Itunes is crashing

My dear cousin, who is a Windows IT person, messed with my iMac (10.6.8) in November, and since then it has not worked right. I lost my ability to run any of the MSFT Office suite--which i have spent 20+ hours trying to fix and have given up. But now the issue is he logged into my itunes store...so when i log him out, or try to access the store, itunes crashes. I tried launching in safe mode, no difference, i removed the one itunes add-on i had.

Please help, I need my mac back!!!

Thanks!