Assuming you have removed Vista and are just running XP, boot the XP disk
and enter the recovery console by pressing 'r' when prompted. Logon as
admin, then from the prompt run fixmbr followed by fixboot.
Actually, you have to change boot.ini because fix boot does not overwrite the
MBR and reads boot.ini to determine what to write to the boot table. Change
boot multi (1) to (0).