If you ever end up with a totally broken .NET Framework install (say, after the installer for .NET Framework 3.5 SP1 falls over), then head over here and get a copy of Aaron Stebners .NET Cleanup Tool:

http://blogs.msdn.com/astebner/archive/2006/05/30/611355.aspx

This is a tool of last resort and let me tell you things were pretty grim and last-resort like last Friday night for one of our production boxes (and me). This got me out of a huge hole where both .NET 3.0 and .NET 2.0 were so badly broken, the .NET 3.5 SP1 installer fell over mid flight, that I couldn't even repair/uninstall from Add/Remove Programs and I couldn't even run the redistributable installers to do a fix/replace/repair.