OK, good to know that the Windows XP box is fixed and working. Your Vista box, is it 32bit or 64bit?
Next thing to try to do (as a test), regardless of the 32bit or 64 bit for Vista.
Do you have the User Account Control enabled?
If so, we will disable it to see if something in the UAC is causing the problem
(please note that once you disable it, it will prompt you for a reboot once you click ok).
How to disable the UAC:
1) Open Control Panel.
2) Under User Account and Family settings click on the "Add or remove user account".
3) Click on the user account that you using for Itunes.
4) Under the user account click on the "Go to the main User Account page" link.
5) Under "Make changes to your user account" click on the "Change security settings" link.
6) In the "Turn on User Account Control (UAC) to make your computer more secure" click to unselect the "Use User Account Control (UAC) to help protect your computer". Click on the Ok button.
7) You will be prompted to reboot your computer. Do so when ready.
To undo the action above and re-enable UAC just follow the same process as above and put the check mark in checkbox and reboot.
