.NET frameworks can become corrupted. This page:
http://blogs.msdn.com/astebner/archive/2005/04/08/406671.aspx
provides a tool which, even if the framework cannot be uninstalled, will remove all .NET frameworks and registry entries, and most thoroughly.
This is the solution to a problem with the newest QuickBooks, which is very .NET-dependent. The symptom is that it appears to start, and then stops, without error messages at all. In this case, remove .NET frameworks entirely using uninstall or the above tool, and then redownload and reinstall the .NET frameworks.