OK, this is a pretty simple setup.
Logon to your router's web based config utility by connecting it to the computer via LAN. Type 192.168.1.1 or 192.168.1.2 or whatever the IP is in the browser. Change it to 192.168.1.2. Now unplug it.
You will need to configure the adsl modem in autodial mode (works best in this mode for this kinda setup). Connect it to the desktop as explained above. Type 192.168.1.1 or 192.168.1.2 or whatever the IP is in the browser. Change it to 192.168.1.1. Correctly enter the DNS etc. whatever needs to be edited to configure your modem in autodial mode. Refer to the airtel and modem manual for this. Now unplug it and connect it to one of the 4 LAN ports, NOT the broadband port, on the router.
Now set the IP of the desktop (the wired LAN adaptor) as 192.168.1.3, subnet as default given, gateway and DNS as 192.168.1.1 and connect it to one of the remaining 3 LAN ports on the router.
Set the IP of the desktop (the wirelss LAN adaptor) as 192.168.1.4, subnet as default given, gateway and DNS as 192.168.1.1. It should now detect the wireless network and connect to it. Entery the security key if you haf selected any.
Set the IP of the laptop (the wirelss LAN adaptor) as 192.168.1.5, subnet as default given, gateway and DNS as 192.168.1.1. It should now detect the wireless network and connect to it. Entery the security key if you haf selected any.
Thats it