With everyone going wireless it’s a good idea to keep an eye on who is using your network, this great little app does the job brilliantly and as it’s a stand-alone piece of software it doesn’t require installing.
http://www.softperfect.com/products/networkscanner/
Before you hit “Start Scanning” you will want to choose Options > Auto Detect Local IP Range and select the network adapter which is connected to the net.
Don’t forget the Router will show up in the list also!