Geez, don't disable wireless security or bother with WEP. Just this week some folks demonstrated a way to crack in in 3 seconds (not an exaggeration). Instead, unless you have a pure-n network (most of us don't), do the following:
Go to the Airport Utility and double-click on your base station. Select the Wireless tab. Next to Radio Mode is a drop-down box; Option-click on that drop-down box. Doing so reveals several additional options not normally visible, including the one you need: "802.11b/g compatible" Select that one, then Option-click on the Wireless Security drop-down box and select "WPA Personal."
These settings limit you to 54 mbps on your wireless LAN but you don't have to give up strong security, you still get great range and very good speed. Until M$ updates the 360 to fix compatibility with n-capable devices, this solution works well.