Computers can either be used "stand alone" or connected to a central network computer (a server).
If it WAS connected to a central network computer (server) before you got it then the userid and password to allow you to logon would be held on the central computer (server), NOT on this computer.
Was this computer used "stand alone" before you go it, or was it connected to a network server?
And now you have got it, is it now connected to the server or being used stand alone?
If it is NOW being used "stand alone" then probably the userid and password information is NOT held on that PC so it wont allow you to logon.