Have a read of that link. Also, here's the thread related to that link:
http://www.cableforum.co.uk/board/showthread.php?t=8435
You should set up the router - & any wireless adaptors connecting to it - to use WEP, or, even better, WPA. Also worth using the wireless MAC filter function of the WRT54G.
The "More" buttons on the relevant tabs in the WRT54G admin interface explain more (under "Wireless > Wireless Security" & "Wireless > Wireless MAC filter").
EDIT: Sorry, they aren't called "Help" - it's the "More" buttons on the right of the admin pages.. Above changed to reflect this.