Hmm, the repair job you tried may screw this idea up, but you can normally restore a previous registry via the recovery console from the "system volume information" folder, so essentially you're doing a limited system restore. Did you try this already?
Also, I'm assuming that when this machine tries to boot, it gets past the BIOS, then when it should be showing the XP loading screen, you're just getting a black screen with a flashing cursor? If it is failing here, I might be inclined to manually transfer known-ok files like ntldr.
Btw, I steer well clear of using the windows XP repair install option, but perhaps that's because once all my other ideas fail, this tactic won't work either. I've had it get me into far worse jams than I started with