Question : How to locate a wireless signal thief

I can see from my router log that it give an IP adress to a computer which is'nt mine.
I'd like to know if there is anyway software or hardware to find out where the computer is.

I know its MAC adress, the IP I give to him but I can't find his computer name or see it on my Windows network.

The user is probably savy cause I've change my WEP key from 64bits to 128 and he was back on the network in 30 min

Answer : How to locate a wireless signal thief

It's not too hard to locate a wireless thief provided you know someone who can do RDF (Radio Direction Finding).  Many HAM/Amateur radio operators do this for "sport" and it doesn't take much time.

You can even do it yourself.  Get a highly directional WiFi antenna and a WiFi card for a laptop with an external antenna connector.  Use a tool like NetStumbler that will give you real-time signal strength readings.

Take several readings at various places around your location and note the direction of the peak signal.  Plot these on a map of the area and you should see them "triangulate" to a point (more likely area) near where the perpetrator is located.

Then call the police!  What this person is doing is almost certainly against the law.  Give the police your evidence, logs from the access point, when you changed WEP keys, etc.  Should be an iron-clad case...