PCHelpware is free and pretty good.
http://www.uvnc.com/pchelpware/
It'll work through firewalls and via routers. You can create a customised version of the server program that will only talk to one specific viewer IP so it's more secure.
I email the server to clients and get them to run it. Gives you full control.
Worth a look if none of the earlier ideas do the business.