Q: how do I change the name of an iMac I bought second hand?

Just bought a second-hand iMac G5 PPC (for minimal use for volunteer work, etc.)  I know I can't upgrade any further, but I would like to rename it permanently so I don't have to delete the original owner's name that keeps popping up and put in my "user" name and password.  How can I get rid of anything that ins't mine?