Boot off your windows XP setup disk by making your CD/DVD drive the first boot device in the system Bios.
Then press any key when prompted to ,at the blue Welcome to Setup screen , press R to start the Recovery Console.
Now select the installation that you want to access by pressing the correct number(dont just press the ENTER key), type the Administrator password if you have one or just press ENTER if you dont.
At the command prompt, type CHKDSK/R , and then press ENTER.
When finished with the scan type FIXBOOT and say Yes to write a new Bootsector ,type EXIT to leave the Recovery console and reboot the machine.
Windows should now load correctly.