Remember, you must set the VPI and VCI before you can use the Modem. The default setting here is VPI 1 and VCI 32 (setting br_1_32) for Airtel Mumbai. Please ask your ISP for VPI and VCI.
Secondly you have two options to use the modem. Rouing mode and bridge mode. Bridge mode is default and you need to use a PPPoE login software (available at speed.airtelbroadband.in for airtel customers). You may also use the Windows XP built in PPPoE ... just go to Network connections --> add new connection--> connect to internet via broadband. You may also use the custom PPPoE client of your ISP.