I am assuming that you have a wireless router. In mine, I went to the security tab under wireless and enabled MAC address for my router setup. Each machine. PC, Laptop, XBox, etc.. that connects to your wireless network has a unique MAC address. On your laptop network card it has a MAC address, a set of numbers for that card only. As I mentioned I went into the security tab and set only those with MAC addresses that I define may have access.
Also, I setup WEP, an ecryption format, that is defined by a password, again I assume that is an option on your router.
The website for your router will have a ton of info on both of MAC addresses and WEP.
Big Mac