XP Pro display stays on

As some of our older XP Pro (Dell) laptops age, they've started to think that
there's activity and keep the display on, when it's set to turn off in ten
minutes of inactivity.

There's processess running (they're used as cpu servers) but no keyboard
activity.

Is there a known reason for this and can it be fixed?

Turning the monitor off with

#include windows.h
main() {
sleep(1);
SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, (LPARAM) 2);
exit(0);
}

turns it off but only for a short time (time varies with machine).

Most of our machines act normally (so far).

(One "fix" is setting the machine to "do nothing" when the lid is closed, and not
ask for a password on return from sleep, and then closing the lid enough to turn
off the display, but you don't get quick access that way. But fixing the problem
would be nicer.)
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