Default Problems Getting MTP Working

I've had the Nexus 7 2013 for a few weeks now and I'd like to root it. I have the Nexus Root Toolkit NRT v1.8.3, and want to do the Backup procedure before rooting. But it won't reboot my device. So I clicked the "Full Driver Installtion Guide" button and can't get beyond Step 2, Configuring MTP Drivers.

Coincidentally, I can mount the device on my PC in PTP mode, but not MTP, which is more useful to me anyway.

My PC is running Windows 8.1 Pro Update 1.

Any ideas?

Steve