Vista - Remote desktop from Vista Home Premium to XP Pro

I recently purchased a laptop that came with Vista Home Premium. One
of the things I want to be able to do is access my home computer which
is running XP Pro. I have had Remote Desktop up and running on my XP
Pro box for years and access it all the time with no problems from
work or friends houses or wherever. I use a dynamic IP service which
gives me an easy to remember domain name that maps to my IP address.
Like I said, this has always worked with no problems. So now I am
trying to use the Remote Desktop client on my Vista laptop using the
same domain name I always use and it won't connect. If I am in my home
wireless network, I can connect to my XP Pro machine from the Vista
laptop if I specify the XP Pro's computer name and it does connect,
but I need to be able to access it from outside my home network via IP
address. Is there something I 'm not doing? I don't know Vista enough
yet. This is the first time I've used it.

THANKS!!