try using you dvd for windows 7 instead of XP to fix the boot loader. I had this same problem occur a few days ago (duel booting with vista and win RC at the time) and after clean installing the win 7 OEM on vista partition my RC disappeared after being moved to d drive. used the dvd for win 7 to fix the OEM's boot loader and now no problems at all. somehow the installation of the OEM corrupted the win 7 boot loader.
should note that I went ahead and reformated and reinstalled the OEM again to make sure there would be no further problems and there are none.
hope this helps.