If anyone can suggest something that works well with MS 2000/XP, doesn't require too much tweaking on the client side and is fairly resiliant to various firewall blocking, I'd be interested in hearing about it.
Openvpn runs on windows. You would have to create a simple installer to copy the key and a config file to the right locations on the client, but the user can rightclick on the desktop icon and select "start" (or something like that) to open the connection. It is what I use whether the client side is Linux or Windows. poptop, and pptp in general, are distasteful to me.