The problem is that when returning back to the PC a little later on, I find it to have power been turned off - mains is still connected on the motherboard. A small movement of the mouse is enough to get the system started up once again. It tells me that the system is locked and that I have to log in again. After that I am able to return directly back to BOINC.

To me, that doesn't really sound like the PC shut down due to overheating. It sounds more like it just blanked the screen or went to sleep because it wasn't used for a while.

1. Check your Windows screen saver setting: is it configured with the 'blank' screensaver and password protection (ie., login screen on resume)?

2. Check your Windows power management settings: do you have the computer configured to sleep or to turn off the display after a certain period of time not being used, and to also require a password when it wakes up?

If the computer were actually shutting down due to overheating, you'd be getting some sort of error message, not just a normal login screen. And you would probably have to use the power button to turn it back on; just moving the mouse wouldn't be enough to power it back up.

The problem is that when returning back to the PC a little later on, I find it to have power been turned off - mains is still connected on the motherboard. A small movement of the mouse is enough to get the system started up once again. It tells me that the system is locked and that I have to log in again. After that I am able to return directly back to BOINC.

On a new Windows install it is configured to put the machine to sleep after someting like half an hour,
running Boinc isn't enough to keep it awake, you'll need to change to a different power plan, and maybe edit that to your preference,

Claggy