My Nexus 5 was working fine with my Windows 7 desktop, but when i plugged in a Nexus 5X (running Android 6.x) it was not detected at all (in Windows Explorer or adb). Installing drivers did not help, and the phone was detected fine on another Windows 7 computer. Here’s what fixed it – go into Device Manager and see if you have a ‘Portable Devices’ node. If you don’t, check to see if you have a ‘Android Device’ node. If so, try right clicking the child node and choosing ‘Update Driver Software’.
For me, it found some random USB compatible device or something, and then the ‘Portable Devices’ node suddenly appeared, and all was good! Maybe it will work for you!