To see if I could repro this, I built up a set of test VMs running XP SP3 in the environment and installed IE8 in the IE6 and IE7 test VMs, and as expected in both cases I had IE8 installed after the reboot.  I then re-ran the IE8 setup, and it gave me a very odd message – that it was removing “a previous version of Internet Explorer”, rather than trying to install again.  It rebooted the machine after this “removal” finished, and then after logging in as Administrator after the reboot, the IE8 setup continued and installed IE8 again.

Curious now, I recreated the XP SP3 VMs, creating a non-administrative user this time along with the standard Administrator account.  I ran IE8 setup, and as expected I had IE8 installed after the reboot.  I re-ran the IE8 setup, and got the same “removing a previous version of Internet Explorer” message, along with the reboot of the machine after that finished.  At this point, IE on the VM had probably been reverted back to the previously installed version of IE prior to IE8, so I logged on with a non-administrative account to see if the IE8 installation would be retried, and it was not.  Also, the machine had a previous IE6 or IE7 version installed, but any time a user went to open it IE would immediately crash.  Administrators could open IE, but it would not browse to any sites at all (internal or external).

We tried to replicate this on Vista, just to see if it was affecting our Vista users, and it did not repro – instead, when IE8’s setup was run a second time, we got a message that setup had found that the version of IE installed was equal or greater to the version attempting to be installed and that installation would not continue in the IE8.log file.  After debugging the setup further, it appears that because the setup engine on Vista is using WUSA it can’t rollback from IE8, so it logs the warning.

I can’t imagine many people are going to ever run across this in their environments, but it took a good amount of time to figure this out, so I’ll throw this out there so if someone *does* ever run into this, they don’t lose the amount of blood I did while hitting my head against the proverbial wall.

Since I spent the better part of 8 hours getting something that works properly in the environment (including site blocking to a custom server), I figured I’d share it with the users of this series of tubes in the hope it helps someone.

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//
// NAME:        proxy.pac
//
// Original:    http://www.cluberti.com/blog
// Last Update: 14th May 2010
//
// Comment:     Proxy.pac example file for use as an auto configuration script
//              template.
//
// NOTE:        Provided as-is - usage of this source assumes that you are at the
//              very least familiar with the javascript language being used and
//              the tools used to create and debug this file.
//
//              In other words, if you break it, you get to keep the pieces.
//
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

// javascript:alert ("Using PAC file");

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Start of the PAC file - function FindProxyForUrl:

function FindProxyForURL(url, host)
{

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Set proxy values as strings to use during return
// NOTE that if a rule directs the browser to return proxy_block, in this example
// the browser should attempt to visit http://internalserver:80, and will display
// the default web site at that location.

	var proxy_primary = "PROXY proxy.primary.internal.domain.com:80";
	var proxy_secondary = "PROXY proxy.secondary.internal.domain.com:80";
	var proxy_testing = "PROXY proxy.testing.internal.domain.com:80";
	var proxy_block = "PROXY internalserver:80"; //<- useful for blocking sites
	var proxy_no = "DIRECT";

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Some proxies may have issues with mixed-case URLs, so change host variable to
// all lower case, and resolve the host IP to reduce DNS lookups later:

	host = host.toLowerCase();
	var resolved_IP = dnsResolve(host);

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Master blocked sites list
// NOTE: Add all sies you wish to redirect to proxy_block here:

	// if destination is .someblockeddomain.com, return proxy_block:
	if (dnsDomainIs(host, ".someblockeddomain.com"))
	{
		return proxy_block;
	}

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// If URL has no dots in host name, send traffic direct.

	if (isPlainHostName(host))
	{
		return proxy_no;
	}

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Master proxy bypass list
// NOTE: Add all other proxy bypass entries into this section:

	// If destination is to an RFC 1918 IP, return direct:
	if (isInNet(resolved_IP, "10.0.0.0", "255.0.0.0") ||
			isInNet(resolved_IP, "172.16.0.0",  "255.240.0.0") ||
			isInNet(resolved_IP, "192.168.0.0", "255.255.0.0") ||
			isInNet(resolved_IP, "127.0.0.0", "255.255.255.0"))
	{
		return proxy_no;
	}

	// If request matches certain hosts in our IP range, return direct:
	if (isInNet(resolved_IP, "11.11.11.11", "255.0.0.0") ||
			isInNet(resolved_IP, "11.11.11.12", "255.0.0.0"))
	{
		return proxy_no;
	}

		If request begins with "xww", return direct:
	if (shExpMatch(host, "xww.*"))
	{
		return proxy_no;
	}

	// If request matches for *.internal.ourdomain.com/.net/.org, return direct:
	if (shExpMatch(host, "*.internal.ourdomain.com") ||
			shExpMatch(host, "*.internal.ourdomain.net") ||
			shExpMatch(host, "*.internal.ourdomain.org"))
	{
		return proxy_no;
	}

	// If request resolves to .external/.testing.ourdomain.com, return direct:
	if (dnsDomainIs(host, ".external.ourdomain.com") ||
			dnsDomainIs(host, ".testing.ourdomain.com"))
	{
		return proxy_no;
	}

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Hosts/domains to proxy
// NOTE: Add all hosts/domains that should go through the proxy into this section:

        // If request is for *.ourdomain.com and we've gotten this far, further
        // parsing is required:
        if (shExpMatch(host, "*.ourdomain.com"))
        {

                // If request is for .us.ourdomain.com, use primary proxy:
                if (dnsDomainIs(host, ".us.ourdomain.com"))
                {
                        return proxy_primary;
                }

                // If request is for .world.ourdomain.com, use secondary proxy:
                if (dnsDomainIs(host, ".world.ourdomain.com"))
                {
                        return proxy_secondary;
                }
        }

        // If browsing to *.microsoft.com, further parsing is required:
        if (shExpMatch(host, "*.microsoft.com"))
        {

                // If request is specifically for www.microsoft.com, try the secondary
                // proxy first:
                if (localHostOrDomainIs(host, "www.microsoft.com"))
                {
                        return proxy_secondary + "; " + proxy_primary ;
                }

                // Otherwise, try the primary proxy first to other microsoft sites:
                if (dnsDomainIs(host, ".microsoft.com"))
                {
                        return proxy_primary + "; " + proxy_secondary ;
                }
        }

        // If browsing to *.someotherdomain.com, further parsing is required:
        if (shExpMatch(host, "*.someotherdomain.com"))
        {

                // 172.16.0.0/255.255.0.0 IP address block -- try primary proxy first:
                if (isInNet(myIpAddress(), "172.16.0.0", "255.255.0.0"))
                {
                        return proxy_primary + "; " + proxy_secondary ;
                }

                // 172.31.0.0/255.255.0.0 IP address block -- try secondary proxy first:
                if (isInNet(myIpAddress(), "172.31.0.0", "255.255.0.0"))
                {
                        return proxy_secondary + "; " + proxy_primary ;
                }
        }

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Advanced proxying based on computer IP Address via the myIpAddress() function
// NOTE: Add all IP Address ranges you wish to proxy for in this section:

        // If the computer's IP address is in the 172.16.0.0/255.255.0.0 range, use
        // the the primary proxy Monday through Friday:
        if (isInNet(myIpAddress(), "172.16.0.0", "255.255.0.0"))
        {
                if (weekdayRange("MON", "FRI"))
                {
                        return proxy_primary;
                }
        // If this check is done on Saturday or Sunday, this will fall through and
        // parsing will continue.
        }

        // If the computer's IP address is in the 172.31.0.0/255.255.0.0 range, use
        // the secondary proxy from January to June, and September to December:
        if (isInNet(myIpAddress(), "172.31.0.0", "255.255.0.0"))
        {
                if (dateRange("JAN", "JUN"))
                {
                        return proxy_secondary;
                }
                if (dateRange("SEP", "DEC"))
                {
                        return proxy_secondary;
                }
        // If this check is done in July or August, this will fall through and parsing
        // will continue.
        }

        // If the computer's IP address is in the 192.168.0.0/255.255.0.0 range, use
        // the testing proxy from 8AM to 6PM local time:
        if (isInNet(myIpAddress(), "192.168.0.0", "255.255.0.0"))
        {
                if (timeRange(8, 18))
                {
                        return proxy_testing;
                }
        // If this check is done before 8AM or after 6PM, this will fall through and
        // parsing will continue.
        }

        // If the computer's IP address is in the 10.0.0.0/255.0.0.0 range, use the
        // testing proxy:
        if (isInNet(myIpAddress(), "10.0.0.0", "255.0.0.0"))
        {
                return proxy_testing;
        }

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Default return direct rule

// If the request matches no other parsing rules, return direct:
else
        // javascript:alert ("PAC finished parsing for: " + host + "\nFrom IP: " + myIpAddress() + "\nReturning: " + proxy_no);
        return proxy_no;
}