THe gateway is your desktop PC, therefore you need to make sure the desktop has a fixed IP Address. This fixed IP should then be entered as the default gateway on your laptop. Also remember that you need a firewall on the desktop PC, it is not being protected by the firewall on the router.
To change the IP address:
Click "Control Panel"
Click "Network Connections""
Right click your network card and click on properties.
Now scroll down until you see "Internet Protocol (TCP/IP)" And highlight it.
Now click the properties button and tick the