Unable to access Vista machine with new username from another PC

> Hi,
>
> 1. I installed Vista Ultimate on a machine called MarkIVista with a
> username "Mark". "Mark" is the only user on the machine and therefore is an
> administrator.
> 2. From other machines, I can access MarkIVista by logging in as
> "MarkIVista/Mark" and entering the password.
> 3. I then changed my username from "Mark" to "John"
> 4. I now log into MarkIVista as "John"
> 5. From other machines, however, "MarkIVista/John" doesn't work. Oddly,
> "MarkIVista/Mark" STILL works. For example, when I want to use MarkIVista's
> printer or share files, I still need to use the old username.
>
> My question: how do I make it so I can log into my "MarkIVista" machine
> with my new username, "John," from other machines? It seems my old username
> "Mark" is still set somewhere.