Post interessante per chi ha problemi a farsi riconoscere i device tramite MTP con windows 8.1 (che ho personalmente testato con un TF101) ma immagino si applicabile anche alle versioni precedenti.
"Another way of fixing this on Windows 8: This problem may happen because you have the Google ADB driver from the Android SDK installed. Windows will pick the ADB driver over the MTP driver, even when USB debugging is turned off on the Nexus 7. It also comes back when you upgrade from Windows 8 to Windows 8.1. To fix this:
1. Plug the Nexus 7 in and make sure USB mode is set to MTP
2. Run devmgmt.msc
3. Locate the ADB driver, which may be under "Android Devices" or "ADB Devices"
4. Right-click on it and select "Update driver software"
5. "Browse my computer for driver software"
6. "Let me pick from a list of device drivers on my computer"
7. With "Show compatible hardware" checked you should see two drivers under "Model":
* "Android ADB Interface"
* Either "MTP USB Device" or "Composite USB Device"
8. Select "MTP/Composite USB Device" (that is, the one that isn't "Android ADB Interface") and click Next.
9. Device should now appear as an MTP device."