I have a problem with adb on Windows 7 and Asus Transformer (ISC, 4.0.3)
adb devices shows empty list.
I've installed Asus PC Suit, USB debugging is switched on. On Linux everything works fine.
if the usb package of android doesn't work , try this : http://support.asus.com/download.aspx?SLanguage=en&p=20&s=16&m=Eee+Pad+Transformer+TF101&os=32
also , make sure the usb cable is short enough . some devices and some computers have problems with too long usb cables . in addition , try different usb ports .