This may be naively simplistic, but can't you install it on a separate partition?
That should work fine? You WILL need to pick a partition
scheme that is compatible with both, however. (GUID) Winders gets more complicated because it really doesn't like to see anything beyond the 4th partition on a drive, and the EFI loader counts as one, so last I looked the best you could do is triple boot if winders was one of them. Mac OS X doesn't seem to care at all, I've partitioned out to 19. OS 9 was limited to 16 I think.
