The Problem might be because C-drive generally contains the master boot record. When you switch on the PC, select the option to boot from CD drive. That can be done by either entering BIOS and making the change or there is a direct key shortcut (F2/F10/F12) depending on ur PC.
This will allow you to boot from a CD.
Now boot from an XP disk, run a system repair. I think this will do the job.