It seems a very strange thing to be happening, and I would have suggested some sort of malware damage, but this seems specific to your Turbotax and .Net framework installation, and so I assume that uninstalling those could be uninstalled to see what happens.

But perhaps before you try that you could run the System File Checker to see if it can repair any system files. You may need your XP CD handy.

Full guidance here;
http://www.bleepingcomputer.com/forums/topic43051.html

See if that helps.

If it does, I would ensure that SP3 is downloaded and installed, to keep Windows up to date.

Mark