If you kept the video setting from widescreen/480i and moved the Wii to a CRT, here’s how to reset them:
– Power on Wii, give it a few seconds to boot
– Hold DOWN on the directional pad of the Wii remote
– While holding DOWN, press the RESET button on the Wii
The Wii reboots and the screen settings are back to default. All my other settings (wifi, console name, etc.) were intact.
Link (look for ch0mp’s comment). Also, you can switch to the A/V and the go back to the composite cables.