I would try using BOOTICE on XP. You can use it to install the Windows XP boot sector in a rather simple matter.
Simply select the Hard Disk XP boots from, usually the one XP is installed on. Then, select
Process MBR, and choose the one for XP. I believe on some versions, this bootloader will be referred to as
ntldr, whereas others will refer to it as
Windows XP 5.x MBR. Either way, that's the one you want to select. Click
Install/Config at the bottom, click OK in the box that appears next, and you're done!
Also, if for some reason you can't boot from XP, there's a CD image called Hiren's Boot CD, which is based off Windows XP, which has this program pre-installed. Simpy burn this CD, boot from it, and use the menu in the Tray area (the lower-left corner of your screen), and you'll find it.