If you are refering to the "guest" account I don't believe it is necessary to edit the registry to remove that account. From the Start menu, click on your user account. It should be an icon or a little square picture at the top of the Start menu. I don't have Vista in front of me at the moment, but try that and look for the options to "edit" or "disable" other user accounts.
You might also try right clicking My computer>click Manage>and when Computer Management pops up, in the left pane, click on "local users and groups". Under that, click on "users". Then on the right side of the screen, right click on the "guest" account, click properties, and put a check in the box that says "account is disabled", save your settings and that's it.
If you are using Vista Home Premium as you said, the "Local Users and Groups" option should be there.