It depends on which version of Windows your father has. If he has anything other than Windows XP Pro, then I would second the suggestion to use LogMeIn.
If he has Windows XP Pro, I get quicker response times from the other computers using Hamachi to establish a VPN connection and then use the Windows XP (any edition) Remote Desktop utility to remote into the other computer.
Hamachi: http://www.hamachi.cc/
Note: Hamachi was bought out by LogMeIn a while back, so you will see the logo at the Hamachi web site, but it is not the same as the other service. All Hamachi does is establish a VPN secure connection ... it is the easiest application I have ever used and works through multiple layers of firewalls and other security software without a problem.
I am also assuming that he has a broadband connection ... I am not sure if either method would work over dialup (even if you could stand the response time it would take to do anything).