1) Check your router's channel number: channel 1 or channel 11 is the best. (I believe it calls the channel in the manual somewhere, but it may not).
2) If your router uses any sort of enhanced power boost, turn it off... for some reason it will not work with the Wii.
Just make sure you have your network name / SSID typed in correctly to the Wii also, otherwise the Wii won't be able to find your network (as well as any passwords).
Hope this helps