Another thing you could do is a repair install for Windows... you will lose your ability to multi boot at this point, but at least it gives you a useable computer.
Then you can try again with another boot loader or figure out what you did wrong with this one.
All the other OSs will remain, just no way to access them until you replace the Microsoft one.
...At least that's my experience with Mandrake about 6 years ago.
