Vista - Remote Desktop Connections

When I open the Remote Desktop Connection application, I am able to enter the
IP address of the computer to which I want to connect.

After entering the address, I'm presented with a dialog box that prompts for
my username and password.
My username credentials are already entered in the username box and I can
only change my password.
How can I get the box to not auto-populate my username? Since I connect to
several different clients with different domains and usernames, this is never
of any use to me.

Thanks.