It isn't vista or your motherboard. It's almost definitely the keyboard.
According to the OP, he's tried different keyboards. If this is true, then it's a dinger of a problem.
@DarkRoy: I know you've said you reformatted recently, but not during this current session of troubleshooting. I know it sucks, but my suggestion is to try reformatting again and do a clean install of windows. If the problem persists, it would seem to narrow the problem down to the motherboard itself.