Problem solved - Eureka!
I still don't know how or why it happened after a clean installation of Windows 7, and I have never experienced the same problem on any other PC I have fixed, but I finally solved it.
I had to disable Windows search via Add or Remove Windows Features in the control panel, then reboot the computer, which brought up and error message telling me that there was no program to run the culprit file/program/script - whatever it was.
I then ran CCleaner and cleaned the registry, which found an unused extension called "search.ms", which I removed.
I rebooted, reactivated Windows Search, regenerated the search index, and the problem was finally gone.
Thanks for trying to help me guys. If anyone else ever gets this same problem, I'd be happy to try and help.
Hope you all had a good Easter.