Originally posted by hulugu
Sounds like someone didn't just war-drive you (drove or walked by) but actually sat down and worked on your network. You must have WEP and you might want to consider enabling RADIUS which effectively hides you network so unless they know its there by knowing its name and the password they can't access it.
Futhermore, make sure your password is not a dictionary word or something easily found using a brute-force attack. Try a phrase or a password that has numbers, characters, and words.
Also, your admin password should follow these standards, but be more easily remembered.