First try to create a new account and see if you have the same problem there. If not, it is likely just a preferences problem. First try quitting the Dock from Activity Monitor. It should start right back up again. If that doesn't fix it, go into ~/Library/Application Support/Dock and delete all the DB files. You should probably quit the Dock again or, even better, log out and back in.
You will have to type ⌘ G in the Finder and type "~/Library/Application Support/Dock" to get to that folder. Bummer Apple, it sure didn't take long to have to unhide the Library foolder, eh?