Start with the Device Manager and see if the device is recognized there. It should be under "Sound and Multimedia".
If it is recognized, but indicates that the drivers are not installed, you need to copy out the Adapter information for the vendor. Then go to the Vendor's site and find drivers for the model and windows version.
If it isn't recognized and you know the adapter to be an installed card, you may need to shutdown, open up the case and get information about the vendor directly from the card. It may help to remove the card, reboot and then shutdown again and reinstall the card. This may force Windows to redetect it.
You can also try installing Everest Home edition (it's free) and in many cases it will provide necessary information and links to vendor sites.
http://www.lavalys.com/products/download.php?pid=1&lang=en&pageid=3