If you have a Windows XP CD, than boot up your PC from CD, when you see Windows setup page chose repair with R button, you are going to see the folder 1.Windows XP Professional or Home, you should chose 1
give the administrator password and when you at the DOS version of C drive, write this command and your operatin system will be fixed.
CHKDSK /R press Enter.
It will take a some time to fix your OS but be patient. At the end when this fix command is finished just type EXIT and press Enter.
Cheers.