Here's the fix:
As I mentioned above, I recently changed the original user account on this computer to a limited account, and added another Admin account. I did this after reading about internet security issues, and saw a recommendation that general internet browsing should be done under a limited account. Anyways, I changed my original account back to an Administrator account, and removed the Admin account, and now sfc runs fine. kind of wierd though that it wouldn't let me run it from the defautl Administrator account either.
Thanks for all your input.