I\'ll be debugging a site in Visual Studio 2010 and editing an *.aspx or *.ascx file, and without warning, it will lock up so that I can\'t edit it
Have you and your collegue got Microsoft Office 2007 or 2010 installed? Run the Office setup from CP > Programs & Features and do a Repair. Reboot afterwards, or at least log out.
I'd understand if you don't want to do a full reinstall of Office, but if the repair doesn't fix this, I would suggest it.