Windows 7 has a built in system (can't remember the name) which I found "just works". The helpee computer generates a code and the helper just types it in. Worked great when trying to help my Dad sort something out. Normally he's pretty self sufficient so I didn't have anything previously setup. Remote Assistance I think it used to be called.

Remote desktop is OK, but the person you are helping can't see what you're doing so if you're trying to show them how to do something, it's no good for that.

No firewall changes necessary.

I've also used VNC with the "listening" client at my end with appropriate port forwarding. You just need to ensure it's installed since doing that over the phone with a non-techie isn't always the easiest.