I would download a Recovery Console CD...burn a bootable CD...and then try the fixboot and fixmbr commands.
Download Recovery Console, Bootable ISO - http://www.webtree.ca/windowsxp/repair_xp.htm
Active@ ISO Burner - http://www.ntfs.com/iso_burner_free.htm Fixboot
Writes a new partition boot sector to the system partition. The fixboot command is only available when you are using the Recovery Console.fixboot C:
(where C: is the drive on which XP is installed)Fixmbr
Repairs the master boot record of the boot disk. The fixmbr command is only available when you are using the Recovery Console.fixmbr
A new master boot record will be written to the boot device, which is the drive on which your primary system is loaded.
If an invalid or nonstandard partition table signature is detected, you will be prompted whether you want to continue. Continue.