Need Help With OS Username Correction!

I have recently acquired a Mac Pro first generation, with OS 10.5.8. It has two hard drives. When I rec'd the machine, I changed the password of the old administrator, using that persons password. I also changed the name of the computer. Well, no matter what I do, I cannot change the username <short name> that appears when I click on the happy face on the task bar near the dock. I tried using a program called "ChangeShortName" and it comes up with the message ... "there is no user account named <the account username that appears> in the NetInfo Database. Can anyone help me out. I have to transfer various files and applications from another computer that is currently running on 10.4.11. I have alot of software and files on the new computer that I do not want to lose if I have to reinstall the system. If anyone can help I would appreciate it.