Ahhhh! Vista!
If it were XP and if you had an install CD/DVD, you would be able to do the "start an install" trick. You pretend you are doing a fresh installation (but don't go that far). After you pass the "do you want to repair or install?" option, saying "install", it will chunter for a bit and then tell you it's found a previous installation, and ask if you want to repair that. You do. Thus far, that's always worked for me (many times) with XP installations.
I suspect if you had a Vista CD/DVD the same would apply - but since I don't allow Vista near my machines, I can't be sure.