I'm guessing that your eth0 is setup to receive the IP information via DHCP. With that in mind what you need to do is set it up with a static ip address and other related information (gateway, DNS etc) and then setup a DHCP server in Linux to listen to eth0 and have your Windows machines set to use DHCP for their IP addresses
Here's a really basic howto
http://linux.cudeso.be/linuxdoc/dhcp.php
Asking questions is not even remotely stupid - not asking them is.