I have a Sonar recording studio and use USB interface units (I have a Roland Octa-Capture).
When you installed it, from your description it sounds like you first installed the unit and then later installed the drivers
. Usually, you have to first install the drivers and then phsyically connect the unit for proper operation.
However, before you do a new install, go to the Sound/Recording panel and RIGHT click in an Empty area in the panel and if not checked, check the boxes to show disabled and disconnected devices and then see if the Tascam shows up in the recording panel.
If that did not help, connect the unit then go to the Device Manager and RIGHT click on the entry for the Tascam (assuming its really installed and listed in the Device Manager) then LEFT click "Uninstall". DO NOT check the box to uninstall the drivers. Then physicall disconnect the Tascam and then restart the PC. After Windows starts, connect the Tascam and Windows should recognize and install it. This may take care of the problem.