I discovered today that OpenVPN 1.2.1 builds and runs almost perfectly in the Cygwin environment under windows. Of course there's no tun or tap device yet, but by running with --dev null and --ping 1, you can put OpenVPN through many of its paces including crypto and UDP network connectivity. I was able to connect to a linux peer using SSL/TLS mode and send pings back and forth.
So it looks like the OpenVPN code itself is ready to go on windows as soon as we can get a tun or tap driver working. James
