said by hrickpa :

how about some one cracking the WEP key to gain access
i am running 802.11b my neighbors keep getting access
when i change the WEP key 24 hours later there back in
linksys befw11s4
WEP 128 bit
been going on for 4 months
they started getting bandwidth greedy that's how i found them
leaching
i have a program that warns me if there is a computer that is an intruder (not belong on my network}
the program gives me a network map of all computers and devices attached

said by TimmyDanny:

You should configure your router to only accept your MAC addresses.

Of course, your neighbor might possibly already know the MAC addresses for your equipment (since they've snooped your network) and be able to pretend to be one of your computers, but it's worth a shot, and simple to do.