macbook pro admin or root name change

[umarr777]

I need help i bought a used macbook pro 2.5 ghz core 2 duo notebook.
the password has been changed but when i tried to change the user name through mac hd in users the popup the can't be deleted or modified required by mac osx...
How can i change this to my name and the admin password .

Thanks
P.S. no installation or recovery cd /dvd

 

[ThirteenXIII]

create a new admin account? then delete the old one?