To get your nexus 7 to work with debugging in windows 7,
0. go to settings -> about tablet -> tap build number 7 times to enable developer options.
1. Enable debug mode on your Nexus 7. To do this, go to Settings, Developer options, and check Enable Debug mode.
2. Plug in the device via USB to your computer.
3. Slide down the notifications bar and Click the notification that says “Connected as a media device”
4. Select Camera (PTP)
5. After switching to PTP, windows will see the Nexus 7 as an unknown device in device manager. Select it and choose the naked universal driver you can get here: