Windows before "vista to hell" used boot.ini on the 1st hd / boot partition.
Grub is stored and referanced from /boot/grub/grub.conf and using a text editor in linux can edit this /boot/grub/grub.conf file to the latest way you would like it using a live dvd to boot from or the hd boot if you can get into it.
At boot by pressing "e" you should be able to load the correct info as well and have it boot this , this works sometimes.
best option is to write the boot to the actual hd mbr sabayon is on and then edit xp pro's boot.ini if it has issues or you fell more comfortable with this.
Vista best of luck , search the forums top right of this page has search + advanced search for "Vista" and should have the work-arounds.
Or as root in terminal grub-install if the /etc/fstab and /etc/mtab is correct to show current and or the way you want the hd's to be read.
The specific code as you mention would be found in some of that links as it is specific or similar to other codes that gets used in general but depends on your config and only you can fix this to the correct way.
Unfortunately this is gentoo based so it is a bit tricky and a bit of reading can help but unlike "Junk Loads another broken window" OS it differs from one to the other install.
Sry if I sound agro not at you , I just had a ebay try to fix a problem session AARRGGHH.
You did not mention if XP or Vista as both have different tricks , you can also use "acronis disk director" as this has a os boot selector or boot loader program that can start this install for you. LILO once installed could also work have not tried it in the last 6 months.
Acronis disk director demo download link:http://www.tucows.com/preview/365026
Hope this helps. Try the wiki on top of this page as it has lots of helpful guides from our users as they got stuff to work.