i need to change my MTU setings for xbox live

depends on the ISP and the router,

you included neither,

what should happen, if your on vista , press start > Network > and your router should appear on that list along with the machine your using and any other devices on the network,

click on that page and it will open router page

web address for router settings:

192.168.1.1

Not sure if it is the same for xbox but that is what you type in the address bar for a regular pc.