I'd recommend VNC (in any variation, tight, original, etc) over RDC.

The full blown version is available at http://www.realvnc.com/. It has as it's advantage, over RDC, that you access the real, live desktop on the remote machine, and you can seamelessely start or end a connection to that real desktop.

RDC gets you to a second desktop, so you and your parents can be both logged onto the same machine, but each of you is hidden from the other. With VNC, you can watch exactly what the remote user is doing (because you see his desktop), and since you can also control the mouse and keys, he sees you move the mouse, click, etc.

Look in the FAQ for how to get to a client (you) to server (your parents) connection through a router, if your parents use one.