It could be a bad sector which houses a vital part of the Laster Boot Record or maybe the disk has become unmountable. Either way, an answer lies within your XP CD, assuming you have one.
Pop it in and if it doesn't run first time, restart and tap Delete to Function 2 on startup to access the BIOS menu, then seek out the Boot Order or Boot Sequence and change it to CD first. Save the changes and exit (usually Function 10) and you should see a message to "Hit and key to boot from CD". Do that and the file copy process runs through. Press Function 8 when prompted to accept the terms of the EULA. At the enxt stall of the screen, select R for Repair and you will be offered the choice of which system to repair. Usually it's Number 1 so type 1 and press Enter. When the Command Prompt appears, type chkdsk /r, hit Enter and wait while a five stage process runs.
Then at the next prompt, type fixmbr and hit Enter. Accept the warning and when that completes, type fixboot and hit Enter again. One more warning and you can type exit to leave the system and restart.
Hopefully, it will work - if not post back for details of curing the unmountable disk.