1 - Press Alt-Ctrl-Del to start the service page.
2 - In the bottom right corner of the page, click on the power button, then, while holding down the Shift key, select "Restart".
3 - Start Safe Mode.
4 - Open the system menu, and navigate to Windows Adminstrative Tools->Services and click on it.
5 - When the Services plug-in starts, locate the "Connected Devices Platform Service" item and either double-click it or right click the item and select "Properties".
6 - Open the "Startup type" menu and select "Disabled".
7 - Press "OK", then reboot your system.
After following these steps, your computer should start up normally.
This service exists in builds prior to 17134, but is disabled by default. There is very little information about what the service actually does, but it appears to have something to do with IoT. Why is it now enabled by default? Ask microsoft.
This bug doesn't seem to be happening much right now, but with 17134 now in the Release Preview ring, I suspect it is going to become very common. I have three systems, and they all suffered from this problem.
The thing is, I used these systems for a couple of days after installation before the bug manifested itself, so a working installation doesn't necessarily mean that you're safe.
This has been submitted to Feedback, but with RS4 in Release Preview, I don't know that Microsoft will be able to fix it before the public release, and even if they fix it in a Cumulative Update, it will be useless, because you won't be able to get Windows Update to run.
I am posting this now because, again, I suspect that it may become a major issue, and I wanted to put a solution out. Good luck, all, I hope that you don't experience this problem. It is nasty.