The computer is in use and has been locked out, appears when it locks itself out, working in a Domain, since the domain control the user permisions. Have you joined a Domain?? or are you trying to run permisions from another pC???
Also you might want to try this:
Go to control Panel and Chose administrative tools, then select Computer management and select from the tree User and Groups management (or just Go to Start/Run and type MMC, and later add the user and groups management snap-in)
Now check to make sure that the rights to the users are set up right, and that the accounts exist in there. Check also that the accounts you want to access are not locked