If you already use a wireless encryption protocol key (WEP), which is a password to log on to your wireless network, you are not completely safe, since an advanced pc user perhaps can bypass it.

You can see if someone is logging onto your wireless network by checking your network log. If there are more devices connected than you have allowed on your network, you have WiFi thieves. You can see this also by checking your router’s DHCP client table, which lists the machines on your network. If you find more machines than what you’ve set up, someone is stealing your WiFi.

First of all, don’t forget to setup a WEP key. For better security, use a protocol such as WiFi protected access (WPA).

To be more safe, set your router’s DHCP to the manual setting and assign each of your computers’ physical addresses. This will allow only the computers recognized by the router, i.e., those you have defined explicitly.

Try also to turn off your router’s SSID broadcasting. This will make your network invisible.