WinRar is the answer
You stumble across this post on your own, but let me save you some time if you don't. For any problems not allowing you to upgrade iTunes (whatever version), open the installation file using Winrar. Go to Winrar's site (google it) and download the software. Then, in winrar, open up the folder where you downloaded itunes. Locate the iTunessetup.exe file and double-click that.
This should make iTunes install perfectly. I have had to use this method for both iTunes 5 and now iTunes 6 upgrades.