If you really don’t want to trash the partition then I’d suggest running an AV. Try PC-Cillin if you have more than one computer you’d like to throw it on. If you wanna go the free route, try link AVG free. Run the virus scans after updating and all that. Secondly grab link ccleaner (download the slim build) which will help remove any cookies or any other garbage files from your computer. I’d run the registry scan on it as well, but make sure you back it up first. Next, grab the free version of link adaware and update and run that. After that reboot and see if it’s any better.
I agree with the above posters that you really should toast it and start anew, but it’s your world so go for it. I understand starting from scratch is a pain, but I’d rather start over than have it affect me for a week or two and end up having to do it anyways. You should also, just do your browsing in OS X instead of xp. You’ll be safer and better off that way. I’d also suggest running xp as a virtual machine through bootcamp as the above poster does, that way you can keep your workflow going while playing games. HTH.