Three different HP desktops have been doing this for a few days now. Nothing has changed. Two of them are only a few months old while the other is several years old. All run the latest patches for Windows XP via autoupdate. Power settings are all set for 'always on', hibernate is disabled, BIOS power settings are either nonexistent or disabled. I'm baffled.
I recently became aware that the Windows power settings are per-user (which is insane, but let's not go there), so I double-checked while logged in as all the usual users: all good.
These machines do get used for random web surfing by staff here, and while they've been told not to do anything stupid (and we have a fairly sold antivirus solution), I'm worried that something has infected them. But whoever heard of malware that just randomly makes PCs go to sleep?