I thought I'd take one more kick at the can here by way of asking a couple of questions that may (or may not) help us get a better handle on your problem.
1) Are you receiving the message shown in
this article in the Apple Knowledge Base or something else? If something else, would you post the exact message.
2) What version of iTunes are you running (iTunes
Help menu >
About iTunes) and is it possible that the problem began after upgrading to this version?
3) Does using the
Vista Program Compatibility Wizard provide you with any helpful information?