Unlocking computer's XP

When starting I get an UNLOCK COMPUTER screen. It
says, "The computer is in use and has been
locked." "Only Chuck or an administrator can unlock this
computer." Often I can just click OK and open for use.
About every third time it gives a screen that says not
correct password. Even inserting my password it says
incorrect. I can then get on only by shutting down the
computer repeatedly until clicking OK works. HELP!