I've tried every suggestion in all of these threads so far & nothing worked. Even after I deleted the ADB drivers, windows 7 would automatically reinstall them. When I tried to update the drivers, win7 said they already had the best drivers (ADB).
Here's what worked for me:
I had to turn off USB debugging, delete the ADB drivers in Device Manager, then reboot the device while it's still plugged in. Windows recognized the device as an MTP device & installed the drivers before it even fully booted.
Phone: Nexus 5 ROM: Stock Kernel: Stock
Tablet: Nexus 7 2013 ROM: Stock Kernel: Stock