Change unknown administrator password

I have a Dell Dimension 2200 with a Windows 2000 operating system.  The computer was used at a company on a network and was given to me for home personal use.  I want to load some games but get an error message about security clearance while loading some of the files.  I want to logon under the administrator to avoid the security issues of my username account or change/find the password of the administrator.  I do not know the password of the administrator.  The logon has a username, domain, and password required for logging on.  I appreciate any help on this matter.  Thanks