My name is XXXXX XXXXX I'd like to help you with your iTunes problem, but I do need a little more information. You said you tried the "usual upgrade prompts" does this mean when iTunes says a new update is available you agree to upgrade? Have you tried upgrading any other ways such as using Apple Software Update or downloading iTunes directly? Let me know and we can proceed from there. Thanks.
Yes I think that should be the next thing to try, as when you install it there might be an error message that is thrown which will point us in the right direction, or it might install without a problem.
Here is the link to download iTunes direct from Apple. This is a link for iTunes 9.1 which is the latest version.
Please let me know if you have any further questions and don't forget to press the green "Accept Answer" button so that I may be paid for my service. Thanks.