Logmein.com is that you need free and awesome!
The setup for logmein won't take you longer than a minute and you can access your pcs from anywhere in the world. Windows has built remote desktop but it blows, I can guarantee anything you find will require you to install a program of sorts.
Logmein is fast and easy to setup. Use their trial. You get 30 days to use it (iirc). It's the best. Really. Don't fumble around with the built in Windows Remote Desktop app.
Yeah, naively I was thinking that I would just put in some port number or something and be magically connected. Since all I really need to do is set up her outgoing email servers, I think she can handle a walkthrough on the phone.
Whatever works best. With logmein, in a matter of minutes, you can log into your mother's computer and have full access. In my experience, it's been a lot easier than trying to give instructions over the phone. You just log in and take care of everything.
It works on both xp and vista
It works with Vista. She would need to install logmein in order for you to access her computer. That's the only catch. If you can figure that out, you log in with her info from your PC and do all the necessary work.
The install process is not bad, if I walk someone through it I usually go through the same steps on my pc so I can see exactly what is on there screen to help them.