Go into System Preferences > Spotlight. Add your entire hard drive to the Privacy section. Wait a couple minutes, then remove it. This should force it to re-index.
If it doesn't, you should delete the index file. Its name is .Spotlight-V100, it's an invisible file, and it should be in the root drive path. You'd need a utility that can see invisible files and delete/remove them, such as
WhatSize. I think it's shareware, but it should have at least enough use time to take care of this job for you.