There have been a number of problems related to IE8 breaking Visual Studio 2005/2008.
Examples:
http://stackoverflow.com/questions/926522/ie8-visual-studio-error https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=426428 http://dpotter.net/Technical/2009/05/upgrading-to-ie8-breaks-debugging-with-visual-studio-2005/
Are there still specific issues with Visual Studio preventing you from installing IE8?