Thanks for your response.

I just found out that it's indeed my Windows settings that cause this behavior.
If anybody can tell me what kind of drug the M$ developers were using when they decided automatically turning a system back on after a few minutes (and with no network attached) would be a good idea pleas tell me what it was. I WANT THE SAME!!

Definately another "WTF?" moment.

My notebook is multiboot, where Windows is hardly ever used and in all my naivity I never even thought of looking in that direction. Next time something strange happens I'll have a look at my Windows installation first.

Thanks.

Hmm, sorry, looks like I cried victory too early...

Wake on Lan is disabled (also "on battery") in BIOS.
Wake on LAN (and wireless) has been disabled in Windows, but laptop still restarts 5 minutes after shutting it down...

Help is still much appreciated.

I have gone through all tasks in the scheduler, but couldn't find any that would start de system.
Since I installed Windows 7 I never touched the task scheduler. Are there any default tasks that I didn't notice but could start a powered down system?

Can the Windows task scheduler interfere with a system that was shut down by another OS? Most of the time I run Linux.

Well, by now it looks like it's my Linux installation.

Shutting down from Windows my laptop stays powered off.

Shutting down from Linux I get the impression that the ACPI power-off call somehow tempers with a wake up timer... Shutting down to "power off" causes the system to boot again five minutes later, while shutting it down without power off (and then manually using the power button to power off) leaves the system powered down.

This is going to be a very instructive exercise...

Time to contact "the community" for some help.

Thanks for your time.

A power off on a notebook is not a mechanical power down, but a state where the system can still be woken up by a few events (like wake on Lan/keyboard). One of these events is the RTC timer. For a reason that is still unknown (at least to me) this timer caused my notebook to restart five minutes after shutting down my Linux sessions with a "power off".

As I see it now the RTC timer was set at every shutdown. To what value it was set I don't know for sure, but looking at the data I was able to figure out it looks like the RTC timer would be set to a moment in the past (being the time the system just booted). Maybe this resulted in the timer thinking it had missed an alarm, and restarting as a result?

I wasn't able to get this issue solved, but I was at least able to find a work around: As soon as Linux boots it now automatically sets the RTC timer to a date two years ahead. Not a beautiful solution, but well... It works...