First you have to Setup the modem as per the instructions given in the user manual. I personally suggest you to get a modem that the BSNL recommends, otherwise the configuration might not match. Once you are done,
(1) Go to the 'Start' button
(2) Click on the 'Control Panel'
(3) Select on the 'Network Connections' by double clicking on it.
(4) There you would find "Local Area Connection", so right click on it .
(5) There click on the Properties
(6) From there tick in the TCP/IP in that box
(7) Again Click on the "properties "
The fields that appear there, Enter these details:
Give the IP Address which has been set as 192.168.1.x where x is your exclusive number.
The Subnet Mask would look like this 255.255.255.0
The Default Gateway would be like this: 192.168.1.1
Then feed in the following DNS nos. which is specific for you in the place or town you live in. You can ask the BSNL for the specific DNS numbers of your region.
The Preferred DNS would be like this: 126.96.36.199
The Alternate DNS as this : 188.8.131.52
Once this part is over you can Click on the "OK" button after you carefully confirm all the fields. You have to confirm by Clicking on "OK" button once more.
After this stage you need to Restart your computer.
Then Open your browser and go to this : http://192.168.1.1/
There when you would be prompted for, Enter these details carefully.
(These username and password would be provided by the BSNL for the first time, you can change the password later on)
Now Click on the "SETUP" on the menu at the top.
now go to the "New Connection" on the left. There when prompted, fill in these details :
Name - BSNL
Type - PPPOE
Username : The same username that BSNL has given you.
Password: The same password that BSNL has given you.
Against MTU : here you have to feed 1452 replacing the default 1492.
If you don't see it get modified, it is most likely that the secure sites will not work.
Against the VCI : Feed 35.
Leave everything else at their default values. Once done, click on the "Apply" button at the bottom of the screen.
Then go and click on the "TOOLS" on the Menu given at the top.
There select "System Commands" on the left, choose "Save all"
and then "Restart".
Make sure that you stay online when you open your browser. Hope that helps.