I help friends and family with technical issues. I would like to administer one of their PCs remotely - owned by a handicapped user who can't implement my verbal suggestions from the telephone. It is probably my communication inadequacies. The PCs at this location are on a DSL router (wireless and wired) with NAT and DHCP. How do remote packages like VNC and Radmin deal with routers and NAT? I looked at their Web sites and they don't address this directly.
Depending on the router/firewall in use, all you may have to do is configure the router/firewall to allow the port(s) used by either VNC or Radmin to be directed to a specific address on the private side of the firewall. Some firewalls can do this, others may require that a public IP address be used in order to direct the ports to a specific address.
When making this kind of opening on a firewall, try to specify the remote IP addresses that will be allowed to pass through the firewall on the ports you are allowing. If you can get a fixed IP address from your ISP, that will help to not open the barn door any more than necessary. You should be able to find the ports used by either Radmin or VNC on their respective Web sites.
If getting a fixed IP address isn't an option, see if the router that your friends/family are using has VPN functionality built in. If that is an option, this will allow you to come in over a somewhat more secure connection without opening any ports in the firewall beyond what the VPN connection should require (the firewall should handle this automatically for you). There is at least one open source VPN client that you should be able to use besides the function that is available in Windows XP if that will be the operating system that you will be using at your end.
Another option is something like GoToMyPC or Webex. Although these are commercial services, some of the set-up hassle is taken off your plate and it lets somebody else deal with that. There will be some costs associated with either of these services, but it frees you from having to become proficient in one or more firewalls that you may not deal with on a daily basis.