You'll need to setup ssh "tunnels" on your linux box that will take traffic on port 5901 and send it through to the appropriate port on the windows XP machine. A command like the following should do it:
yourmachine> ssh -X -f username@machine2 -N -L 5901:yourmachine:5901
Then, you would fire up vncviewer:
yourmachine> vncviewer localhost:5901
As long as you've got vncserver running on your parents' box, this should do the trick
See
HERE for more info