RDP client through VPN doesn't use domainname

> Hello all.
>
> I'm in the following situation:
> I work remotely and want to connect to my server, therefore I've set up a
> VPN. So I connect through a VPN from my Windows Vista to my Windows 2003
> SBS and then I want to connect to my RDP Desktop. I start my RDP client
> and I have 3 choices:
>
> a) 192.168.1.4\username
> b) other user
> c) smartcard
>
> I use the firsst choice and type my password, not being able to remove the
> IP address indicated nor change the username. Obviously the server tries
> to authenticate me as "192.168.1.4\username" on the domain, which is
> failing. After the timeout has passed, I get the login error and only
> *then* I can change username (put it to "username") and type the password
> to get logged on.
>
> How may I change this behaviour?
>
> Thanks
>