As of the time of this writing, Google does not provide a USB driver (for Windows) for the Nexus 5. The usual link (http://developer.android.com/sdk/win-usb.html) shows Wind
There are multiple hardware revisions of Nexus 5. So, the accepted answer doesn't work for all devices (it didn't work for me).
Open Device Manager, right click and Properties. Now go to the "Details" tab And now select the property "Hardware Ids". Note down the PID and VID.
Download the Google driver
Update the android_winusb.inf with above VID and PID
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&**PID_4EE1**
Now in Device Manager, find Nexus 5, and update the driver software, and browse to the location where you downloaded.
The driver should be installed, and you should be see the device in ADB.