The version of grub currently in portage has a bug when multibooting with that other OS (win), rewrites the paritition type tables, etc. The newer version of grub (I think its .92) corrects for this bug and a few others. Not sure if this is the proper avenue for the suggestion, but can portage be updated with the new grub version?
Somebody with a bit more grub experience ? I am a lilo boy ;-) If nobody up to it, I can do it, but am not too sure about the needed patches, etc.
I submitted an ebuild for grub-0.92 (bug #4020) and would be glad if someone could test it.
*** This bug has been marked as a duplicate of 4020 ***