in this instance i would resort to using the windows xp recovery console, here is how:
1. Insert the Windows XP CD-ROM into the CD-ROM drive, and then restart the computer. Click to select any options that are required to start the computer from the CD-ROM drive if you are prompted to do so.
2. When the "Welcome to Setup" screen appears, press R to start the Recovery Console.
3. When the Recovery Console menu is displayed, a numbered list of the Windows installations on the computer is displayed (usually only one entry-c:\Windows-exists). Press a number before you press ENTER, even when only one entry appears. If you press ENTER without choosing a number, the computer restarts and begins the process again.
4. When you are prompted to do so, type the Administrator password. If the administrator password is blank, just press ENTER.
for a full list of options / commands that can be used from here type HELP and hit enter, for info on a specific command type HELP COMMANDNAME and hit enter.
I would try two options, 1st would be to try he "fixboot" this writes a new boot sector on the system partition.
if this didnt work then try "Fixmbr" this option repairs the boot partition's master boot code. The variable device is an optional name that specifies the device that needs a new MBR; omit this variable when the target is the boot device.
I hope that this is of some use to you? i would be interested to know the outcome.