You won't be able to do a repair install if you can't boot in.
It is very easy to replace the registry hives from the backup by booting to the command prompt in Vista.
Unfortunately, I haven't figured out how to do it on 7 yet.
I think a reinstall is the quickest and easiest way now.
If you need to save stuff that's on there , either:
1. Download this freebie from Paragon - it burns to a cd - boot from that and use the file transfer wizard to copy anything off you need . FREE Rescue Kit Express - Free Download!
2. During the reinstall - when you get to the screen asking where to install, do not go to Drive Options (advanced), do not format anything - just click on the partition you wish to install to.
You should get the option to save everything to save everything to a Windows.old folder.
Hope it helps