It would appear that Microsoft likes to keep files needed for critical core functionality of .NET, in folders called ‘temp’.
I ran into an issue where contents of a bulging temp folder were deleted, in turn destroying all installed versions of .NET. Windows Update began failing, and the ability to update or reinstall .NET was completely lost through the conventional means of Add/Remove Programs, ect.
I was about 30 minutes from buying a Mac, until I found a Nuke button for .NET. This nice little tool is called ‘dotnetfx_cleanup_tool.zip’.
After running this tool which completely erases all traces of every version of .NET, you can reinstall .NET through Windows Update.
I also found a nice tool called ‘Windows Install Clean Up’ that will wipe out any non-.NET application, if you’re having issues with the Add/Remove Programs function of any application in your Windows control panel.
Both of these tools weren’t particularly easy to find (Microsoft removed the latter from their site). Hopefully this helps someone else in a similar situation.