Why Vista hibernates itself even if computer is set to be always o

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Why my Vista hibernates (or sleeps) sometimes by itself although my desktop
computer is set to be always on and hibernation (and sleep) is turned off
already since installation of Vista, ~20 months ago?

This computer has been running several weeks or even months without turning
off or without going into power save mode. But recently it has started to go
into hibernated (or sleep) state. Today, after being away from home for 5
hours, leaving the computer unattended, I found the second case, the first
was some weeks ago. In the first case when the computer hibernated by itself
I did not understand at first what was happening, why the screen was blank
and only computers power led was blinking, and I forced restart, after what I
found hibernation file on my system disk. I checked power saving options and
turned all of them off again. Today I found the computer in this weird power
saving state again, tried to resume, but without success (there was some disk
activity after I touched the power button, but the screen remained blank and
the power led was still blinking). So I hold the power button for 5 seconds
until the computer turned off, waited for 10 seconds and turned it on again.
In my surprise the computer resumed exactly the same state as it was before I
left home, opening all my documents that I was working with. Exactly like
after resuming from hibernation. I checked my power options again, but every
power saving option was turned off!
I totally hate if OS tries to do something on its own. I want that if I
turned off every power saving option and timers, then computer stays always
on. Why such things are happening? Is there some keyboard shortcut that
changes power management options if I accidently press it?

BTW, my computer is UPS powered and no power event has occurred according to
the UPS log. My UPS is not even controlling power management of this
computer, it just provides battery backup in case of power failure. Another
computer (Linux OS) that is using the same UPS is controlled by the UPS and
keeps also the logs.