I've got my Nexus 5 connected to my PC right now. I'm trying to copy files off of it, but the Nexus doesn't realize it's connected to the PC. With Android debugging on, Windows realizes there's an "Unknown USB Device" connected, but Android doesn't recognize the computer is connected at all. I've tried the other computer in our house and had the same issue.
I'm running on 4.4.2, so it shouldn't be something too out there, but I can't figure out what else to do.