Try running sfc /scannow. Start/run/ type: cmd/enter/type: sfc /scannow (note the space after sfc). You will be prompted to insert your xp disk.
Also install, update and run Ad-aware and Spybot search and destroy. Run them both and let them fix anything they find.
You could also consider a full install of xp.
Keep us updated on your progress
You can get the above programs from
www.download.com or
www.majorgeeks.com