accessing the net through an ethernet requires NAT I think...or "masquerade" as it's often called. so check your kernel so that it contains iptables and everything in it you need...after this, grab an app called iptables for your distro so that you can use this kernel-feature, and you're ready to build up a firewall and a NAT that lets your boxes access the net through the server. you'll find a lot of FAQ:s and examples in the homepages of iptables...
also, if you want your windows machines to be able to use some medias on the server, install samba into the server and use it to share...NFS works for linux-linux-filesharing, I think, but I'm not sure if samba handles this too...but for windows' samba does the thing.
iptables are quite easy to use, after you just read a bit about them and get the grip of it
after this, just create a nice script that configures your firewall/NAT and after running it, along with samba services, you should be able to use net and access files on server with your machines. check these sites:
iptables.org
samba.org