Since I built my pc I notice that as soon as I turn on the xbox with the network cable plugged in it kills my internet connection. I've not tested it but I'm pretty certain anything else on the network would kill it as well.

The cable modem is plugged in through USB.

Help before I buy router.

Depending on your provider they may have mapped your internet connection to a MAC address, and seeing as how your X-Box doesn't have the same MAC address as the one in your PC, it probably won't work, some map it to other things like domain name etc. Although I'm not really clear on how your hooking up your USB modem to your non-USB X-Box unless you did a mod so you could use USB

it would help if u tell us your network set up.. fully, how is your pc connected to the net? how is your xbox? .............