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
I had similar problems as people here with Nexus 5 on Windows 7. No .inf file edits were needed, my computer was stuck on an old version of the Google USB drivers (7.0.0.1). Windows 7 refused to install the newer version even if I tried to manually select the directory or .inf file. Had to manually delete specific cached .inf files in WINDOWS\inf folder, follow directions here: http://code.google.com/p/android/issues/detail?id=62365#c7
Also be sure USB debugging is turned on in developer options. There's a trick to expose the developer options, click 7 times on the build number at the bottom of the "About Phone" information!