What does the Device Manager show? For instance, when the tablet is in the recovery menu, can you see an unidentified device?
EDIT: What operating system are you running? Are you an administrator? Are you running Impactor as administrator?
Apple iPhone 4S (64GB, black, iOS 7.1) ■ EVGA Tegra Note 7 ■ Apple MacBook Pro Retina (128GB, i5, 4GB RAM, late 2013)