If you do a repair install you lose all the xp data. It guts out xp and rebuilds its. You will have to reactivate also. You will also have to download all the updates, again.
A much safer way to fix system files is to run sfc /scannow. There is a space between ths c and the slash. Have your xp cd in the drive.
Start
Run
sfc /scannow
It may take a while to run. Don't worry.