I have been using Visual Studio 2005 Beta 2005 for a few weeks and one day, completely out of the blue it failed to open my project. I tried to open another one - the same result. I also couldn't create a new project.
Unfortunately, I didn't jot down the error message, but I remember it mentioned Microsoft.Common.targets file.
I checked this file on my machine and it turned out that it was empty. I did some googling to make sure that it is not normal and then reinstalled .NET Framework 2.0 (use repair option) - it helped!