There are thousands of posts about repairing Windows XP so use google about...
"XP Repair Install" and "XP SFC SCANNOW" for starters.
The problem is that repair can be so time consuming that I tend to use the backup copy.
Bob
Recently, hopefully, cured what ailed my computer but now it's far more sluggish than it ever was. It's like the bullets have been pulled but the holes remain. Any way to get my system back to the way it was pre-infection other than a full-re-install? Thanks!
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic