I've never used Windows 8, but I couldn't get my Nexus 5 to show up at all on Windows 7.
Here's what fixed it:
1) With the phone plugged in go to device manager (Run: devmgmt.msc)
2) At the top there will probably be something along the lines of "Android device", expand this and you'll get "Android Composite ADB interface" (or something similar, can't remember exactly)
3) Double click the ADB Device, go to driver & click uninstall, when a box pops up tick the box to delete drives & press ok
4) Unplug & plug back in the Nexus 5. The drivers should install again. Now if you go to device manager the Nexus 5 will appear under Portable Devices
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