If you're seeing that error, what's happening is the web server (running an ASP.NET page/app) is erroring out when trying to either process your request or provide the page/app you requested. IE8 still sends the post-platform registry information as part of the user agent string, and most ASP.NET apps and pages don't like very long UA strings. An easy test is to install fiddler
, click Rules > User Agent > IE8 (Windows 7), and then try to hit a page with a new instance of IE8 and see if it fails - fiddler acts as a proxy and changes the default UA string with the one you just specified. If it now reliably continues to work with fiddler running after making this change, then you'll know that's the reason. There are ways to fix it, including (but not limited to) installing IE9 (which no longer sends the post-platform header information from the registry as part of the UA string).
If you want to see what your current UA string is, visit the following page - it will be displayed at the top of the page in large red font: http://www.fiddlertool.com/ua.aspx