if your are refering to the repair option of winxp pro, yes, i have done that plenty of times and it leaves your files, programms, and settings untouched. it simply corrects win sys files. you will even still have your desktop pic. how to do it?
easy: boot your pc with the winxp cd (or run it as an update from windows), when prompted the 1. time to repair, choose the new install option. then it will find your windows partition and will ask you again if you want to repair. this is when you choose "repair". it acts like it's newly installing everything, but dont worry, everything will still be there.