Download dotnetfix_cleanup_tool: The current build was created on April 29, 2010.
Download full versions of NET Framework 2 SP2, NET Framework 3 SP2, NET Framework 3.5 SP1 from Microsoft.
Do not install Frameworks 1 or 4, they are useless for you and your XP.
For more information about the .NET Framework cleanup tool, see the following links:
Install Cleanup Tool and remove ALL .NET Frameworks one by one then reboot.
Reinstall Frameworks in the order of 2, 3 and 3.5. Do not install 1 or 4.
Then everything will work for you again.