.NET framework corruptions are rare but they do happen from time to time for various reasons, commonly though the reason I mentioned above (windows update gone wobbly).
There are a number of ways to fix .net installation errors.
[1] There's a repair option in Add/Remove programes
[2] There's a manual method as explained in MS KB 306160 'How to repair an existing installation of the .NET Framework'
[3] A free .NET Framework Cleanup Tool to Uninstall And Fix Corrupted .NET Installs.

Some more info at Aaron Stebner's MSDN Weblog