Found this on another forum:
This is how to install BSNL NIC(u100) in windows 7:
1.Get the u100 windows 7 driver software from the BSNL website.
2.Select the 32-bit or 64-bit version as per your OS version.
3.Select the USB modem installer in the folder. Then the U100 installer window opens.
4.Insert the USB modem when asked in the window. Then it will either show 'please install drivers manually' or 'installation complete'.In both cases, click finish.
5.Now right click computer -->computer management-->device manger.
6.Click other devices. Inside other devices, there will be two folders with name 'Data Interface' showing caution signs. Right click one of them and click update driver software.
7.In the window which comes next,click browse and select U100 driver windows 7 32 bit/64 bit(as per your OS version) from the location where the folder is installed. Then click next. A window appears next showing driver successfully updated.
8. Do the same with the next folder named 'Data Interface'.
9.Now Enter the folder Modem which appears in the desktop. In the dialler window,wait for the USB modem to get ready and show the range. Then click settings, type in your username and password and save it.
10. Now you can dial and get connected.
BSNL's website is at:http://www.bsnl.co.in