It always feels like starting over when developing for a new device. If you’re developing apps on Windows for Android and using Google’s Nexus 7 tablet as your hardware test platform, you’ll need to download the USB driver from Asus. It is not currently included in the Android SDK so it has to be obtained from the Nexus 7 hardware OEM, in this case Asus.
I found it there by searching on Nexus 7 on the Asus site. Click on the returned result and then select Android as the OS in the dropdown box. After downloading, open the zip archive and just follow the OEM USB Drivers installation procedure on the Android Developers site.