Hi Nikhil,
It's really easy, nothing too hard. First of all do you have the Airtel DSL Connection connected to your USB, I assume that it is connected through USB. Now you need to buy one more Lan Card or USB LAN.
Next connect the sify cable broadband to one LAN Card, put the IP address on it it was previously and then disable it by right clicking on that particular Local Area Connection.
Next buy a Crossover Cable (to connect one PC to another PC), it should be less than 80mtr, i.e. the distance of your both PCs should not more than 80mtr. Now put one side of the crossover cable to your server PC LAN Card and another side to your 2nd PC LAN Card. Put the IP address of your 2nd PC as IP address=192.168.0.2, Subnet Mask= 255.255.255.0, Gateway=192.168.0.1, DNS= 192.168.0.1.
Now in your 1st PC right click on Airtel DSL dialup, go to properties page, click on advance tab, click on Share and select your home Network and press OK. And you are allmost done.
When you want to connect to your friends through sify Cable, simply just enable it by reght clicking on that perticular Local Area Connection.
If you requir any further help you can contact me through email.
purnendu1001@yahoo.com