Problem solved, but I still don't know what was wrong.
I was using a dvd drive, and so was the backup I checked. Here's the stupid thing: I removed the dvd drive from my computer, installed a new one (not the second one I checked) and that did the trick. The other two drives seemed to be working, but I guess not well enough to satisfy the Windows 7 dvd.
Anyway, the third drive booted and I installed the os. Thanks for the suggestions!