Dottore Rivets,
You seem to have backed yourself into a corner with that machine.
My inclination would be to acquire and fit a second hard drive (they are not expensive), then copy all your "stuff" over to it - documents, downloaded software installers, that sort of thing. Then make sure that you have install CDs for the software you want to keep.
Remove the new hard drive (for safety). Wipe the existing detritus-filled one and run FDISK to set up the partitions the way you want them to be. Then install your new Win 7 (full version) on it. Then plug your new hard drive back in, install your software from it or the install CDs, and copy stuff to the partitions you want.
You can't upgrade from Win 7 RC to Win 7 final using an "upgrade" disk - or I don't think you can. I've not tried.
PS: Biushfiva's trick certainly worked for installing later versions along the Beta - RC route. I'm not sure what it will do if you try to install Release Home Premium over RC Ultimate - I could imagine some "interesting" results.