Get a free program (free trial anyway) called "Your Uninstaller" and use that to remove the game completely (and cleanly) from your system. Then reinstall it fresh from the original discs. Then apply the official patch(es).
Of course you'll lose any saved games, so if you want to keep those, back up those folders elsewhere on your hard drive before doing any of this.
You could also check the disc to see if it's scratched or smudged, and give it a gentle cleaning (as you would clean any other CD or DVD).