Does the Tascam show up in Control Panel>Device Manager>Sound Video and Audio Controllers? If it does you know the drivers are definitely installed, if not, they aren't. Are you using the latest drivers, sometimes the drivers included are updated by the time you unwrap the product. They should be available from Tascam if there are indeed any.
Are you using a USB hub? If so try plugging directly into USB port on computer. Also try using different ports.
Make sure all your USB controller drivers are up to date. Control Panel>Device Manager>Universal Serial Bus Controllers. Double Click on first one>Driver Tab>Update Driver>Do automatically (Whilst connected to internet). Then repeat for all USB Controllers.
If you haven't already done so, I would update Pro Tools to v9.03. It's a free update to PT9 users and is downloadable from here (Bottom of Page);
http://avid.custkb.com/avid/app/self...p?DocId=409231
Run installer (Do not uninstall PT9 first as installer is an update not a full installer) with PT closed. Re-install Tascam Drivers.
Is there another computer running PT9 that you can plug into?
It has been known to happen; USB cables can be malfunctioning even though they are new so if nothing else is working you will need to verify that it is functioning properly.