Power Mgt: "Turn off monitor" "Never" always reverts to "After 20 mins"

OS: Windows XP Pro SP3
Machine: Acer Aspire L5100-TV
Monitor: Full HD TV connected via HDMI.
The monitor is switched off after 20 mins of no KB or mouse activity;
annoying when watching TV or movie.
No screensaver is used.
Using Power management, setting "Turn off monitor" to "Never" in any
Power Scheme does not help, for some reason this setting always reverts
to "After 20 mins" when checked later.
I don't know if and where this 20 mins value is defined in the registry.
I have searched the internet, but I cannot find a good solution for this
problem.
Someone had made a little program that periodically generates mouse
events, but there should be a more elegant solution.
Suggestions are welcome, thanks in advance.

This is most likely caused by an Acer power management utility running.
Check your system tray (bottom right) for the Acer icons and define your
power preferences there.

Thanks. But since I replaced the Acer supplied Vista with a standard Win
XP Pro, no Acer utils are running.
But maybe it's a setting in the BIOS, I 'll check that.
My complaint is somewhat weirder still, because the 20 mins revert
occurs only after the VLC Media Player starts playing a movie.
So if I no movie is started, the monitor stays on forever.
VLC MP has no power options other than disabling the screensaver.

I implemented the following work-around in the form of an DOS batch file
(monon.bat) that is running during watching a movie.
-------------
NEVEROFF:
REM set "Turn off monitor" to "Never"
PowerCfg /change "Presentation" /monitor-timeout-ac 0
REM wait 300 seconds
sleep 300
goto NEVEROFF
-------------
This seems to work; sleep.exe can be obtained from Cygwin.

[PSH]

Did you ever find out why this happens?

I have the same problem when running videos, that pwr management monitor setting is changed to 20mins