Here:
http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx
is a tool, by a Microsoft rep, which will reliably remove .NET frameworks. Very useful when the uninstallers fail. However, any other .NET frameworks you are using on the machine will probably need reinstalling, and the author recommends the following:
http://blogs.msdn.com/b/astebner/archive/2008/03/07/8108332.aspx
be done first.