Sorry, I didn't read your post properly. (I don't function well until I've opened a bottle or a can and, unless I start on that bottle of port I've been saving, I haven't got any booze in the house!).
Have you got SpyBot Search & Destroy on your PC? If so, it can lock access to Internet Options. Solution here:
http://www.boutell.com/newfaq/browser/internet options.html
Otherwise, if you know that the problem is recent, try running System Restore.
Lastly, you could try sorting the problem out within the registry. Instructions are in that link. However, I advise backing up the registry before running regedit:
http://support.microsoft.com/kb/322756
Chris