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 Windows USB drivers for several of the Nexus's line. However, the "Nexus 5" is not included. Note that the Nexus "S" (letter-S, not number-5) IS supported.
Also, from this site, the latest version of the current download is version-8, July-17-2013. When I compare this with its predecessor, version-7, the files are the same.
Until Google fixes this, what are we to do? How can we debug and develop for the Nexus 5?
I just wanted to bring a small contribution, because I have been able to debug on my Nexus 5 device on Windows 8, without doing all of this.
When I plugged it, there wasn't any yellow exclamation mark within Device Manager. So for me, the drivers were OK. But the device was not listed within my Eclipse DDMS. After a little bit of searching, it was just an option to change in the device settings. By default, the Nexus 5 USB computer connection is in MTP mode (Media Device).
What you have to do is:
I hope this will help!