What you can do is boot up one copy of XP, go to Start-> Run-> msconfig-> "BOOT.INI" tab-> "Check All Boot Paths" button. I've never had this exact issue, though I believe this should fix it. Alternatively, you can create a small partition on one of your HDDs and make a tiny Linux install, which will automatically install the GRUB bootloader. A small Linux install is especially useful when you have to recover data when your windows fails.
Hope that helps!