Connecting a Mac to a Windows XP VPN Server

OK, I have the following set up:

My Computer (Windows XP Home)
...connected to:
Actiontec Firewall / Router

I have set up my computer to accept incoming VPN connections.

I have forwarded TCP ports 1723, 500, 50, and 51 in the router to my computer.

I then followed the instructions on this site to set up the Mac (I don't know anything about Macs, so I dicitated it over the phone to the user of the Mac in Florida):
http://www.wsu.edu/~peckham/macVPN.html

Everything set up fine and worked great setting up, but it doesn't work when we try to connect. Are those the right ports for VPN? And is the PPTP Mac thing capable of connecting? It says it can't negociate a connection. Tomorrow I will try having another XP computer connect to the VPN.

The other obvious question here is, can a Mac connect to the XP VPN ?