Try unplugging and turning off everything. Computer off, xbox off, router unplugged, modem unplugged, and all ethernet cables unplugged.
Leave it for a minute or two to let your hardware forget it's configurations, you might even have to hit the reset button on your router if it stays on even after being unplugged (some do that).
Plug in the MODEM and wait for it to get re-aquainted with your ISP.
Next plug in the ROUTER and give your modem and router time to get re-aquainted.
Now plug your computer and make sure that works.
Lastly plug your Xbox back in and you might have to clear the internet configuration settings but it should build a brand new bridge with your internet.
Some Notes: It may be to late for this but not all modems and routers will play nice with the Xbox or Xbox 360. Xbox.com has a list of routers that are certified to work great, but that doesnt necissarily mean yours won't, it's just a precaution that it's nice to take.
Also, according to the bonus disc that came with Halo3LE, having your modem too close to flatscreen monitors and cordless phones can interfere with wireless signals, but it sounds like youre going wired so that shouldnt effect you, plus I've got my modem next to my monitor and it doesnt seem to bother it.
IF ALL ELSE FAILS CALL 1-800-4MY-XBOX
[sometimes you get idiots who don't know what they're talking about, but if you call back, you might get a knowledgeable person.]