Anyway, last night it installed itself, and for a while I thought it had bricked my desktop. Luckily, I had an iPad, so I was able to search for 'windows 10 upgrade black screen' and similar things and find a huge number of solutions, most of which were completely inapplicable because they assumed you could somehow change system settings while your computer was frozen at a black screen.
One of them told me how to get to safe mode, though -- I had to pull out the plug and reboot three times in a row, and then it would bring up the boot repair options.
In safe mode, I could see the screen, but I couldn't run any apps, which meant in particular that I couldn't run the update app which would let me revert to windows 8.1. I couldn't even run the start menu! I could run the old control panel, though, and IE to do more web searches, and I found someone with my exact machine who'd somehow figured out how to fix the problem -- I needed to disable my unused onboard display adapter.
That worked. Slowly. It still booted to a black screen, but this time it eventually actually booted for real after a while. And not in safe mode!
Then, I started getting the error messages from all the stuff I had installed. Including Steam, which couldn't find the network. 99% of my games are in Steam, and that's something like 75% of what I do with this computer.
At this point, it was 4am, but I clicked 'revert to windows 8.1' anyway. And it worked!
...except that now the first thing it wants to do in windows update is install windows 10, and it's downloading it even though I said 'don't automatically download updates' and unchecked 'update to windows 10' and clicked 'stop downloading' (the stop downloading button, when clicked, disables the stop downloading button but has no other effect).
This is the second try to cancel it. The first try corrupted my windows update database and it needed to be repaired.
It looks like I might have avoided re-upgrading against my will by declining the EULA, though. It's 'cleaning up files' with only an indeterminate progress bar, and in the background windows update is still refusing to show anything but the windows 10 upgrade.
Jesus fucking Christ. x.x