Have you tried a mirror download? Sometimes files can get corrupted in the process of downloading because of the location.

Do you have a recovery CD for Windows 7? If so, you should try a low-level format (ie: zero-wiping, etc) and then reinstall Windows using the recovery CD. This is usually used as a last resort, but I've actually had several conflicts with other programs that didn't even exist when I switched operating systems. If you want a program to do low-level formatting, I would suggest using "Darik's Boot and Nuke".