Yes, VNC is the right answer, and yes, it's easy.
You'll have to punch a hole in the firewall in the router, and set it to forward the appropriate port (e.g. 5900) to the machine. This might also require having a fixed, rather than dynamic, IP address from the router (so the router can always know where to find the machine). But if you have any experience at all with configuring the router this should be simple.
For each machine, you need a VNC server. (Or, if the XP machine is XP Pro, you can use Microsoft RDP).
For the Mac:
OSXvnc
For the PC:
RealVNC
You'll also need a client on each machine.
For the Mac:
Chicken of the VNC
For the PC: The same RealVNC package above includes a client.
Enjoy!