This happened to me couple a days ago using Visual Studio .NET 2003 developing an old ASP.net application in Windows XP Mode. Using product released 9 years ago on a newer OS has it’s charms.
How to fix it: Re install the IDE ( on 2 out of 3 machines this seems to resolve the problem ) or better, upgrade your project to more recent version of Visual Studio and enjoy all the benefits. As for the company I work for we finally did the upgrade ( Visual Studio 2010 )