I'm in a situation where I act as a sort of techinical support to my family and less techinically experienced friends.

I'm looking for a remote desktop solution where it's possible to setup a "zero-install, double click an icon"-solution where the client computer contacts me so that I may interact with their desktop.

The last part is important as the people in need of my help don't know how to configure their router or even the firewall software on their own computer. They are able to click an accept button when asked if a program should be able to make outgoing connections. They have many different kinds of routers, as well as software firewalls, and I rather not deal with the problem of how to connect to them using whatever as well as the actual problem they are having.

It must be:

1. Free of charge for non-commercial use.
2. Possible to use it in a mode where the computer wanting to share its desktop should be able to make a connection to my computer. My computer has a DNS name we can use.
3. Compatible with both Windows XP and Windows 7.
4. Independent of a third party server or infrastructure.

Explanations of the above:

1. I don't want to spend money on it when I help them for free. If it's free as in freedom, all the better!
2. I guess this boils down to being callable like showdesktopto.exe opscomputer.com where opscomputer.com is my computers DNS name. If that is possible then I can create a shortcut they can use to connect to me when they need help. It's nice if it's possible to specify a password or key file which I can use to authenticate myself, but it's not required.
3. They use the OS which their machine comes installed with. That means Windows XP or 7.
4. I want something which will work in the long run. Using a third party service which might not be available when I need it disqualified such solutions.