When you perform a fresh install of Windows on a system you will need to obtain the necessary drivers for all the devices within and attached to your computer so that Windows knows how to talk to them.
Some drivers are generic and do not need to be installed manually while others are either installed automatically by Windows using its automatic update feature or need to be installed manually. For automatic updates to work, you will need internet connectivity. Your modem must have come with an installation CD. If you can find that and install it on the computer, that is the best way to solve your issue.
If not, you will need to locate the necessary driver ( http://driverzone.co...ice.php?id=4181
) , transfer it to the system using a USB drive or something similar, and install it manually.