I'm going to join the fray and take a crack at this. I'll try to help as best I can to resolve the situation in your current setup, but I would like to say that I agree with what others have posted and would say that this is a little (but not too much) unorthodox. Typically desktop, server, application, and personal machines are put behind the router in the IPv4 paradigm. We then use NATing to allow multiple machines onto the interwebs. However I only say it is somewhat unorthodox, as dedicated network equipment is often given external IP addresses. It just look like you are using your Linux box as network equipment and as a personal machine.
I'm going to focus on questions concerning when both NIC's are up, seeing as this is probably the desired end result.
> > When eth0 is up and eth1 up,
> > the Linux box can not access the web.
> > the Win2K box can access the web.
> > the Linux box can not access the Win2K shares.
> > 'ping ATTrouter' fails.
> > 'ping -Ieth0 ATTrouter' works.
What does a traceroute show on the Windows box?
What does a tcpdump or wireshark output of a ping to 99.188.244.1 show on both Linux and Windows?
What happens when you remove the 192.168.1/24 route? Does the Linux box then have internet access? Does the Windows box lose internet access?
A quick way to start to narrow down where the problem or problems exist is to completely disable your firewall and perform your tests again. If any of them succeed then you know it has something to do with netfilter.
I'll try to look at your config more and see if I can spot anything in particular.