Can you boot into VGA mode (safe mode) and log in as Administrator. If so, i would take a guess that if you have programs that startup with NT, such as NAV or McAfee, one of these is causing the PC to hang. Have you tried changing the keyboard. Justbecause you can change a slection at the begining, doesnt mean one of the three keys needed to access the log in box isnt broken. Alternatativley, if you can access the machine remotely, say through PC Anywhere, try doing that because if you cna then ctrl/alt/del (via the menu in pc anywhere) you will know that it is the keyboard.
Hope this helps.