Open your device manager in Control Panel and look for a listing for the Nook with an exclamation point on it. If it's there, right click it and select uninstall.
Also, if you previously had a Nook Color or another version of Nook connected, then it may be looking at the older Nooks drivers.
Oddly enough, my wife just came to me and told me her Windows 7 PC wouldn't connect to her Nook Color. After playing around with it for several minutes, I realized she was using the USB charging cord. I switched to a cord that I use for my camera and it connected fine, but it refuses to connect using the Nook charging cord. If you're using the charging cord and by some chance have another non-Nook usb cord, you might try that.
I had this problem, too, with my nook reader. I changed cords and that didn't help but then I tried a different USB port and that fixed it.