Vista - Changing Admin user name?

I recently purchased a 2 week old dell from someone. When I upgraded to
Windows Vista I noticed the computer has two user accounts. Administrator and
Guest. Guest in turned off by default so obviously I must be using
Administrator. My question is, when I go to Task Manager I noticed on the
Processes Tab the user ame is listing the previous owner of this computer. In
user accounts I already changed the Administrator name to my name. Where can
I truly change the user name to my name and completely get rid of the
previous owners name. I cant find anywhere in windows vista to change this
information. Maybe in the registry??? Thanks