Unable to access Windows XP Professional OS

Recently I purchased a Dell Optiplex SX-270 computer from a business closeout auction. I came to find out that the PC was utilized on the company’s network and it is still password protected. Password is unknown. When PC is turned on, I am unable to get past the logon screen to access Windows. Can anyone please provide any assistance with how I might be able to bypass this screeen or resolve this issue in order to use computer.