I have an Xbox 360 Elite (the original black one with 120GB) and about two weeks ago I started having issues with my connectivity.
I have a Linksys E1000 router by Cisco and ISP of Time Warner. Both to my knowledge have the latest firmware.
Firstly, I have another Xbox 360, the basic from the original launch, and two computers as well as a variety of handheld devices that can all connect to the internet through my router perfectly fine.
The main issue is that whenever the Xbox is turned on, the modem shuts down, all connectivity fails, internet phone you name it. I have Time Warner, and while its been problematic before never like this. The best part (sarcastic) is even if the Ethernet cable is unplugged, turning the Xbox on will shut down all connectivity. There is no wireless adapter plugged in or in use. It makes absolutely no sense and I desperate need help.
It should also be note worthy that my account does have a pass code to sign in and I do have a Kinect but it is not plugged in. Also I have not updated yet to the newer dashboard due to this issue.