Win 7 Antivirus Software?

I recently bought Win 7 for my Fujitsu T4020 Lifebook laptop because I could not get on Firefox or IE and had to purchase it in order to use the Internet altogether. I've since come to find out that maaaaybe that was a wrong thing to do. Was it legit or not? I got a product key code and everything. I since downgraded back to XP PC Tablet Edition for everything to work right (some features weren't working correctly on Win7 Ultimate) and need anti-virus software now. I backed up my old settings, music, photos, etc. and am unable to take the DVDs I used and restore my settings and everything else. I need to know if that was a bad purchase or not in order to attempt to get my $ back. I got a confirmation e-mail, obviously, as well. Thanks!