I'm out of ideas now.
Me too. : - (
Other than ...
Let's try completely removing iTunes and QuickTime completely from your computer as described in
this article in the Apple Knowledge Base.
After downloading and running a fresh copy of the iTunes installer, is there any difference?
If not, all I can suggest for the time being in uninstalling iTunes and QuickTime again and installing an earlier version - your problem, whatever it is, seemingly being connected with your recent upgrade.
The most recent old version available on the Apple Site is
iTunes 7.3.2. . (It's the last version that supports Windows 2000 but it also works with XP.)