What, in a bare XP Pro, runs that persuades XP to turn the display on?

There's nothing awfully suspicious in task manager, but it shows
70% of a dual core CPU active, 50% of which is my own background job
that runs for days; so about 20% unknown. explorer.exe sometimes turns
up in the #3 cpu slot after my job first and Idle second.

Automatic indexing is turned off, no network activity.

It just started. If I turn the monitor off at a 1 minute limit, it
turns off and then turns back on again in about a minute.

"Normally" it runs dark and at 50% cpu exactly.

A couple machines have gone through this episode recently, making me suspect
that it's something XP does occasionally, say monthly.

Even if it does something in the background, why would it turn on the
display?
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