Microsoft now has tools for fixing the .Net stuff.
I have no idea if any of them could help, Just that a listing of all the options is usefull. Plus direct links to proper Install packages.
As always with any of this stuff there are System Requirements, procedures, and some reading to make sure that you have the correct item for the problem.
Microsoft .NET Framework Repair Tool
The Article on it, and its revisions
.NET Framework Setup Verification tool
The Blog for it http://blogs.msdn.com/b/astebner/archive/2008/10/13/8999004.aspx
Updated info on the verification tool http://blogs.msdn.com/b/astebner/archive/2012/09/13/10349253.aspx
.NET Framework Cleanup tool
Artical with must read and downloads. http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx
Error code “0x80070643” or “0x643” occur when you install the .NET Framework updates
Windows Update or Microsoft Update repeatedly offers the same update (some people it was Net updates that did not install) http://support.microsoft.com/kb/910339
Possibly usefull the Microsoft Visual Studio and .NET Framework Log Collection Tool
They listened :-) Myself I used the verification tool, then did not get anywhere. I had that situation where I could not install or re-install because Of a leftover registry or file item that the system believed it was installed. I used a Cleanup tool that tore out the whole thing, and was finnaly able to re-install repair one of my setups that way.
.NET Package Installation Links (mostly verified by me) These should ALL be Microsofts own site links.
Microsoft .NET Framework Redistributable 1.0
Microsoft .NET Framework Version 1.1 Redistributable
Microsoft .NET Framework 2.0 Service Pack 2
Microsoft .NET Framework 3.0 Service Pack 1
Microsoft .NET Framework 3.5 Service Pack 1
Microsoft .NET Framework 4 (Standalone Installer)