Depends how computer saavy you are - it's not a trivial recovery
This is an MS KB article
here which you could follow. You will need your WinXP setup disk - hope you have one.
Alternatively, if you are not confident messing with recovery consoles, you may need to take it to a decent local computer repair shop.
---------- Post added at 13:50 ---------- Previous post was at 13:48 ----------
The last time I had this was due to a disk hardware problem, so it 'might' not be the only corruption.