If your comfortable working on your wireless router. Try doing a Hardreset (hold that little button in the back down for 20 seconds) This will restore back to factory settings. And set it up with no security and see if you can connect.
Also you can take you xbox and try to pick up (not connect) another wifi network.
Any other Wifi devices that you are using?
Another thing you can try is checking for a firmware update for your router.
It can be a hardware issue on the routers transmission side or the xbox's wifi card, but these should help rule out any software issues at least.