Firewall settings

[Mike Campbell]

My 81-year old mother who lives 200-miles away likes to play with her
computer but is always having problems. I need to be able to access her
computer remotely. I intend to purchase a personal license of RealVNC once I
see that I can accomplish what I want. For now I have a free license
installed on both computers and her computer is running as a server.
Firstly, I don't understand all that's involved with port access settings.
I'm hoping someone can get me jump started. Here's what I have to work with:

On my end-
Service, Cox digital
Router, Linksys WRT54G
PC virus protector/firewall, Security Shield 2008
OS, Windows 2000

On her end-
Service, SBC.Yahoo.com
Router, 2-Wire 2701HG-B
PC virus protector/firewall, Security Shield 2008
OS, Windows XP