Windows Setup in QEMU says "The computer restarted unexpectedly" and will not continue

Symptoms

The machine boots from its disk, shows the Windows logo, then a box: "The computer restarted unexpectedly or encountered an unexpected error. Windows installation cannot proceed." OK restarts into the same box.

Cause

The machine was stopped while Setup was still installing, between its restarts (inferred: the disk was last written at the time the machine was stopped, and the 32-bit twin, left running, went on to the first-run screens). Under QEMU's TCG, Setup takes over an hour, so it is easy to stop it early by mistake: quitting QEMU, kill, or a script that shuts the machine down.

What to do

Start again with an empty disk. The half-installed disk cannot be repaired.

rm base.qcow2
qemu-img create -f qcow2 base.qcow2 64G
# boot the ISO again and redo Setup

Then leave the machine alone until the desktop or the first-run screens appear. Check its screen instead of guessing: screendump FILE.ppm in the QEMU monitor. Once Windows is at its first-run screens, a savevm snapshot keeps that point.