For non-experts like you and I, you need a CD disk or a good computer repair shop. They have restore disks.
I had the same problem and the only thing suggested to me was to Google find an i386 file and image it to a CD, run and repair. Some forums have free files to use this way. I didn't feel confident enough to try it.
Since you only had a virus, removing it didn't help? XP has a restore option and a recovery option that work well for most injuries.