.net 2.0 and 3.0 are part of Windows Vista and can't be repaired. All other editions can be removed with the .NET Framework Cleanup Tool (http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx
). Now you can reinstall the .net Frameworks.
Also check this KB article:
When you try to install an update for the .NET Framework 1.0, 1.1, 2.0, 3.0, or 3.5, you may receive Windows Update error code "0x643" or Windows Installer error code "1603"
http://support.microsoft.com/kb/923100/
"A programmer is just a tool which converts caffeine into code"
Want to install RSAT on Windows 7 Sp1? Check my HowTo: http://www.msfn.org/board/index.php?showtopic=150221