Hi AH, welcome to NST.
Do you own a copy of bootmgr ?
i.e. you have a licenced copy of Vista or Windows 7
You'll need to copy bootmgr from the root of your Vista/W7 system into the root of your XP partition.
When you rewrite the MBR to use the Vista/W7 boot instead of XP's, it will look for bootmgr in the boot sector instead of NTLDR.
You have to put a copy there for it to find.