You could boot to safe mode and log onto the administrator account(assuming it doesn't have a password) and then create a new account and make it an administration account. If you don't know the password, you could try breaking it or removing it with one of numerous utilities available on the net.
You could also try Ctrl+Alt+Del keys at the login screen and when it prompts for a username - password you could try logging into the Administrator account from there. But it will more than likely be disabled for standard boot. Safe mode is probably your better bet.
A company PC will most likely be run without welcome screen (windows2000 login look), meaning you should be able to select an admin account to log into (what you're looking for is called "local admin" and it is stored on disk, unlike domain admin password. If not(you see welcome screen), then do as above by fusion_gtx.
If you can see administrator in login, try to use empty password and "password", "Admin, admin, and Root, root. If they don't work, download ERD2007, or one of the pass cracks for XP. They will come as bootable cd image. Burn it to CD (not like data, but like iso). Boot with it and follow instructions (for ERD go to locksmith). You want to remove password, meaning make it empty. Then reboot, login as admin and set your own pass.