This thread looks familiar :)
If you are using a router - then the router will dish out the IP address the router will be 192.168.1.1 and the first PC that connects will be 192.168.1.2 for example and if another pc connected it would be 192.168.1.3 for example.
If your router is set up right, you need to use whatever software the lappy used to connect to the net and NOT the windows one, unless you only have the windows software to connect wirelessly