I have a feeling that the last rule may be wrong. It will need to be -o eth1 (your card going to the windows machine) and it may need an -i eth0 for the card connected to the net. so it will be
iptables -A POSTROUTING -i eth0 -o eth1 -j MASQUERADE
You'll have to look at the man page or on www.google.com/linux
since my skills at iptables are shabby at best. The main structure should be there though.
p.s.Sorry I was no good but sometimes you need to admit ignorance before you can become informed. If you find a solution post it back so we know how to do it in future.