I actually just released a very early beta of my remote pc viewing application. Here it is : http://vizure.net/products/60-remote-php
I basically had the same issue as you. I needed to remotely help family members but typical software wouldn't work for various reasons (campus firewalls, work place firewalls). So I created this and just put out a beta last week. To control a system, you simply put some html on your web server that supports php. Then tell the user to download the executable, install, and hit the big Remote URL button. Then have them enter the http directory you created. Hopefully you can walk them through that (I know how painful it can be).
This approach should work through pretty much any computer that can access the web, is completely free and you can check out the php yourself to see that there are no gotchas. It basically uses the web server as a proxy server and as an added benefit requires no client viewer, just use your web browser. It's an extremely early beta. I tested a couple of ideas for about a week and that was one that panned out pretty good. I can control as all my desktops remotely from any browser, and through my Palm Pre. Just gotta work on copy/paste support, adding an interface and performance optimizations.