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
The one from Google USB Driver worked fine for me on two machines (Windows 7 x64 on both). Once Windows failed to auto-install the driver, I just right-clicked on the phone in Device Manager, chose "update driver" and gave it the path that I'd unzipped that driver into.