Sorry, I've been offline for the last few days.
Since XP is no longer supportd upgrading to Windows7 would be the best option. Soon programs will not be written to run on XP. Anti malware program developers will also stop support for XP.
If you haven't any plans to do too much with this computer in the future you could reformat and reinstall XP. After the reinstall you would need to update to service pack 3. Service pack 3 can be downloaded form Microsoft.
It will say it's for IT professionals and developers but you can safely use it.
•Download ServicePack3 from HERE
•Save it to you desktop
•Boot the computer into Safe Mode
•Once Windows loads in Safe Mode double click the file you downloaded
Either option will require you back up anything you want to save before ugrading to Win7 or reinstalling. You can find a very good guide for backing up HERE.