This worked for me. Hopefully it helps.
While the nook is plugged in (mine showed the 2 "Disk Drives" and 2 Drive Letters under portable devices). Open Device Manager and delete/remove the 2 disk drives called "B&N NOOK Tablet USB Device" (this should also remove the weird drive letters under portable devices). Now go to Control Panel > Advanced System Settings > Hardware Tab > then click "Device Installation Settings". Change this setting to "No, let me choose what to do", and make sure "Never install driver software from Windows Update" is checked below that.
Now unplug and replug your NT, and you should now see an Android Composite device with the "!" next to it. You should now be able to install the custom drivers described in the other posts.
Essentially it seems windows is trying to "help" you by guessing at the drivers you want to use.
If this doesn't work, you can try to remove ALL android devices using the USBDeview Application that you can find http://www.androidsim.net/2009/08/ho...driver-on.html
I used USBDeview for the removal part just because I had a Kindle fire, HTC device, samsung device, all installed and I think they were fighting a little bit. I now have results from adb devices, and dare I say ROOT!?!?!
Hope this helps some of you!