Hi there,
I use a mixture of stuff to remotely support users - if you are having firewall problems, work around them!
Try
http://www.logmein.com - they have a free version which should do just what you want.
FYI I use:
Tightvnc (for the java/web client and the normal client)
Atelier Remote Commander (like VNC, but push installs the server, starts it and then connects to it)
Quest Central (remote cmd prompt)
Remote Desktop (Windows RDP)
LogMeIn.com - works like Gotomypc and is dead easy to talk end users through installation, no fixed IPs or dynamic dns shenanigans either!
Out of that lot, usually I can get a fit with the problem I'm trying to solve - Atelier is pay for, everything else is free!
HTH
David.