The first thing is...holy c**p. That's a lot of networking to sift through.
What I would do first is try to find some way to get the 360 out from behind the Windows PC. The 360 has to make 4 hops just to get outside your internal network. But the other thing is, you have to port forward every device that is between the 360 and the Internet.
But, if you want to make this setup work.... this is what you gotta do. Find out of the Quest DSL modem is also a router. If the DSL modem is a router, then you need to login to the DSL modem and forward the appropriate ports to the WAN IP address of the Linksys router. Then you have to login to the Linksys router, and forward the same ports to the IP address of the 360 I believe. Then, with the network bridged, you may have to open up the ports on the Windows Firewall.
If the DSL modem is just a modem and not a router too, then you don't have to worry about that. The fact the the DSL modem is assigning a NAT IP address to your Linksys router pretty much guarantees that the modem is a router too.
Still though, I would get the 360 attached directly to the Linksys router, then you can just DMZ the 360.
And get your public IP address and your MAC addresses off of your post. People can really screw things up for you if they have that information.