Computer boots when I switch off the lights

I have more details regarding My computer boots up during the night. I got a used halogen lamp with a transformer and can reliably reproduce the following:

• Computer and lamp are running
• I shut down Windows until the computer is "off".
• I switch off the lamp.
• The computer starts its boot cycle.
• I press power switch to shut down computer.
• The computer stays off until I hit the power button again. (until I boot into Windows 10 and make a normal Windows "shutdown", that is!).

I am not really sure what the reason is and what to do about it. Any ideas?

EDIT: To answer the questions from your comments and "answers":

Sounds like there are elect[r]ical problems within your walls causing power to not run as reliably as it should.

The basic problem (the computer starting unexpectedly) has survived the change from one apartment in one village to another apartment in another village, somewhere between four kilometers and three miles apart. The "new" (used) lamp I got for my new desk in my new apartment just made the issue reproducible.

Are they both plugged into the the same power circuit

Yes, they are plugged into the same power strip. I have tested a surge-protector and a normal strip, and I have tried to put them on the same power circuit but different outlets. When I plug the lamp into a different power circuit, the problem is no longer reproducible. Other unknown devices on the same circuit still trigger my computer's bootcycle from time to time.

[After using the power button] It stays off even if you the switch the lap on and off again?

Yes, it does. Only when I shut down Windows, it can be booted somehow from the outside.

Desktop or laptop computer?

Desktop computer. Self-assembled. It did work correctly for at least one year with Windows 7.

Do you have BIOS settings for advanced power management? Like "Restore from AC power loss"?

Yes, and it is already set to OFF.

Sounds like it might be an issue with the power supply then.

I have switched to my spare PSU, the problem remains.

is that light a two prong or three prong plug?

It's a two-prong plug.

A surge protector power strip can bypass superior protection inside the PSU (explains why a spike is seen by a computer's power controller).

I have tested that the behaviour is the same with a normal power strip.

Is the power strip connected to a two prong or three prong receptacle?

A three-prong receptacle.

If three prong, does a dedicated ground wire properly connect back to the breaker box or did some previous person have contempt for human safety?

The wiring in my current apartment should have been made in the mid-90s according to the then valid German VDE. In the previous apartment I had the same problem, the fixtures were originally from the 40's, but should have been completely replaced in the 2000s by a VDE-bound electrician.

Is polarity correct?

Not sure what you mean. Both PSU and power strip shouldn't distinguish between phase and ground.

Is motherboard connected to a baseplate via many conductive standoffs or just one?

About six to nine brass-colored metal standoffs.

An EMF spike that can affect @Alexander's display like that

My "display", a Samsung TV, is not affected by a brown-out. I can switch to TV mode and it runs through without being affected by the lamp, although the lamp is placed exactly between display and computer, with more or less equal distance to both, and the display also is on the same power strip.

does your computer have a remote control to turn it on/off?

The TV has a remote control, but the computer doesn't. The TV would then have to wake the PC via HDMI, but I have not found such a button on the remote. I don't have a blutooth dongle, NFC or WiFi device that I know of. I have disconnected Ethernet but the problem remains.

What's probably happening is this:

• Your PC has a clever feature usually found on servers, but also in several desktops, in the setup, saying something to the tune of: "In case of power failure: [ ] do nothing [ ] stay off [X] last state".
• Your PC's power unit is slightly faulty - probably a capacitor is a bit cooked. When it feels a sudden "spike" in voltage, such as you get when you switch an inductive load, it considers this as a temporary blackout. Therefore it resumes its previous state.
• Windows does not set correctly the "off" state (this is a common APM problem), i.e., the low level circuitry considers itself to still be on. Possibly this is because the desktop is partially on, e.g. to support wake-on-lan.
• when you turn the PC off from the button, it goes to "really off", so even if you switch the lamp again, even if it senses another "blackout", it stays off. That's why the lamp-switching only works once.

You can break this cycle in several ways:

• use some spike suppressor (anywhere from USD 1,99 to USD 19,99) on the PC's power outlet. Note that they're power rated, i.e. if your PC absorbs 700W, you'd better use a suppressor rated at least 1kW, or it can either not work, or positively malfunction. This will probably stop the problem from occurring, whatever the real cause, by simply "hiding" the external spike from the PSU.
• disable "Power Resume" and/or "Wake on LAN" functions in the BIOS.
• there are several voodoo rituals that may make Windows convince the PC that it's lights out, really.
• replace the PSU. Shouldn't be needed, but this behaviour might be symptomatic of something worse coming. If you've got a slowly cooking capacitor, it may pop at some inconvenient time (they usually do).
• set the "In case of power failure" to "[x] STAY OFF!" if at all possible.

Get your housing power checking. Sounds like there are electical problems within your walls causing power to not run as reliably as it should.

Something else you can check is to load up the bios and check the power settings, I can't remember the exact name but there should be a feature along the lines of "Power on computer after power loss" set that to no and it should cause the computer to stop starting "randomly".

Some computers have a setting in the BIOS where you can have the computer automatically turn itself on after a power failure. The first thing to do is check to see if your computer has that setting enabled. If your lamp is causing a power blip when you turn it off, and both are on the same circuit, that would explain why your computer turns itself on.

The secondary issue is, of course, why your desk lamp is browning out the circuit in the first place. That's an electrical problem that you should probably look into. Either the transformer brick or the power strip have a grounding problem.