@JJ
I know this isn't the solution for you (you explicitly said no linux), but since it was so easy (I never thought it'd be THAT easy)... I did this by installing Debian GNU/Linux to the routing machine and:
apt-get install ipmasq
Since I had both NICs already set up, it worked out of the box. For sharing network I installed samba and smbfs (oh, and that was a bit more work).
