Internet Explorer will not open

I have run into an issue several times where windows updates attempts to install IE8 on XP clients while IE is still in use. It has a similar effect to the one you are describing, appearing to corrupt several registry entries and critical IE files in the process.

This will likely have made it impossible to uninstall IE even with the tools available from Microsoft if your problem is related (as this tool relies on an un-corrupted registry). The only solution to this issue that I have found to fix is by downloading a copy of IE from Microsoft using a different browser and installing it straight over the top of any existing IE installation that might exist, which should fix any corruption that may have occurred.

You should be able to locate the latest IE version for your machines here: http://windows.microsoft.com/en-US/internet-explorer/downloads/ie

This may sound self explanatory, but ensure you are logged in as either the local or domain administrator when doing this, otherwise registry files will not be written properly.

Hope this has helped.