you will just need the XP Bootable CD- which should be the same version as what you have currently installed.
Once you boot from the CD, it will show you options to do a clean install or repair. Select repair.
Find details with pictures in the link below:
Thanks for the information. But I do not have the Bootable CD on hand. I have to get it from my niece and I don't know whether she still have it or not. She is now using Windows 7 already.
I will tell you the answer later. Thanks a lot!