run windows xp setup, okay when you have an option to press enter to re install windows(i think) and you have an option to press R to repair windows in recovery console. PRESS ENTER choose you partitaion where all your files are then you will have an option to REPAIR which uses you xp disc and then you will be asked to imput your serial key

you wont lose anythin

i have done this many times for other people, and i think im doing it on mine 2 day
100% it will work