If you have .NET applications crashing on you, use this tool:
http://blogs.msdn.com/astebner/pages/8904493.aspx
and do this:
1. Run the tool, remove all .NET, all versions.
2. Install .NET of the versions you need.
3. Go to Windows Update, get all appropriate updates, especially the .NET updates.
4. Reboot.
Things are likely to work a whole lot better.