I too have a bsnl terracom usb data card and I got it to work in Linux mint 10 which is based on the ubuntu 10.04 that you are using. It's actually quite simple except for one thing. You will have to first uninstall the bsnl 3g application that you have installed in windows xp. Do this after you pull out the data card from the usb slot. Once you have uninstalled it, plug it in and immediately open your “my computer” where the data card will show as a usb storage device. Open it quickly select all and copy to your desktop. Do this quickly because within a couple of seconds the system will start to install the files on your data card and it will not show up on my computer as a storage device. You will then have to uninstall the application and try it once again.
Once you have copied it, you will find a folder inside named PCL_COMNLG.tar.gz. The tar.gz at the end just means that it is a compressed folder very much like .zip except that tar is used for linux. Make sure that you copy this somewhere where you can access it from your ubuntu, like a pen drive.
Restart and log into ubuntu, copy the PCL_COMNLG.tar.gz folder anywhere you like, right click on it and select “extract here”. It will extract all the files. Open the folder you'll find another called tools with two more folders inside. Open the folder deb which will have two more folders inside, wvdial and qt. Both these folders contain packages that need to be installed. All you do is double click them and it will automatically install in your system. Just remember that because of certain dependencies issues you will have to install the packages in a certain order. There is nothing to this other than to double click it and if it throws up an error, cancel but keep it in mind and go back to it at the end. Just make sure that you install all the packages.
An application called Joinair will open up which is the application that you use to connect. Click on the gear like symbol and then on connection settings. Then click on the blue button that says view. Then put in your name in the configuration name, check if the dial up number has *99# and if it does not put it in. click on “use the following apn address” and enter bsnlnet under it. Click ok.
Click on the world icon to take you back. You should now see a “connect to and your name” with a button underneath it. Only if this button is blue in colour can you connect. Many times this button does not change colour. Just keep your pointer near the small tower symbol that shows signal strength under the world icon and right click twice. Usually the color will change within a couple of seconds. Click connect and you will be connected in a couple of seconds.
As soon as you connect I would advise you to do two things. Go to menu, package manager and search for firestarter which is a firewall and install it. If you need help configuring it you can ask here. Also click on the update manager (shield icon) on the system tray and update everything.
Let me know how this works.
There is another way you can connect using ubuntu's own network manager, but it has certain issues. Let me know if you want to know how and I'll tell you.