I had the same problem at first too (having to reset my modem everytime), but after a little while I just "fixed" itself, I dunno if its because i have the network settings in the Xbox on 'automatic' or not, but now I can just unplug the Ethernet cable from my PC and hook it up to the 360. (Quick tip: If you have two Ethernet cables laying around, hook one of them up to your 360, and the other to your PC, and when you want to play xbox live, plug the 360 Ethernet cable into the modem, and unplug the PC one. works well for me)
As far as the USB goes, you can. If you have a USB-to-USB cable you can hook up your 360 to PC via USB, and PC to modem via Ethernet, which will allow you to be online on both at once (I believe). Basically working as a router, but with a router you can actually have your PC\360 more then like 2 feet away from eachother (or however long USB cables are lol).