XBox Live connection problem

[molineux1980]

I bought a 360 yesterday, and after much swearing finally gave up on getting it set up for live. My PC connects to Virgin via a USB Speedtouch 330 modem, which you have to 'dial up'(Even though its broadband) to connect to the net. I plugged the ethernet cable into the back of the computer and into the 360. I now have a shared network, and had to manually input the ip address and DNS settings. Ran the tests from the Xbox dashboard and it passed the wired, ip, and DNS tests, but failed at the MTU stage. Anyone have any ideas? Will I need a router? I'm assuming the modem isn't good enough?