I've scoured the net and tried all the declared fixes, I've read every thread on XDA on this topic. Now I'm turning to the guys who know more about Windows than Microsoft. I realize this topic is not the normal fare around here but perhaps someone has a real solution. I have a Nexus 4, Windows 8.1 MCE. Both work perfectly, but no matter what I try I cannot get the ADB or MTP USB driver to load properly. Windows 8, 7, these methods work fine, not 8.1. There is speculation about the driver signing, but the workarounds I've tried do nothing. Anyone? Thanks!!
I've had no problems with neither ADB nor MTP with my Nexus 7 (2012) on 8.1; I also recall my Nexus 10 and 4 working too when I had them. Do you have a N or KN (not sure which it is, but the one that lacks media stuff) version of Windows 8.1? Perhaps you need that Media Pack thing?
I have MCE and I tried the Media Pack getting the "you've already got this" message. What I get is the little yellow exclamation point with the details telling me that Windows can't find the specified driver, but the correct driver name is in the title. Even if I go through the full manual install method, pointing the right way each step, I get the same message. Barring some insight from here, I think I'll be installing the update next. I know for sure I'm not the only one with this issue, I find it all over the net. I also know Microsoft has said some things about driver enumeration and what sounds like some standards issues. They have released a couple of fixes for USB devices, one of them a Sony phone. So... I'm thinking, maybe?? Maybe the update will include some fixes??
Update: I installed the "8.1.1" update and the issue remains unchanged. (My system does boot straight to desktop now though... only easily noticed change so far.)