Sounds like a drive is failing. When it was happening to me, the system would sometimes be able to "catch" itself and re-start before completely powering down. When I switched off/on it worked normally until the disc completely failed and "locked". Nothing a new drive (hybrid SSD for key/often used elements and traditional larger drive for storage), SSD (if you can afford it), or standard drive wont cure.

Possible that you have a developing short in the power supply and may be seeing surges in the system. But I would go for the easy repair and upgrade first. In either case, this is a warning to back up all critical data to something external NOW.

If the screen actually locks first, it's not power. If it were, you would simply go dark.

I'd check your event log for clues. It might be something as simple as overheating, or it could be the drive. My bet would be the RAM, either your main or your graphics.

Regardless, back EVERYTHING up as soon as possible. Pretty sure you know this, but I agree that it sounds like a drive. Good luck- at least a new HD isn't too expensive.

Backups are up to date as of yesterday.
All drivers are up-to-date.
Memory tests okay.
All fans blown out.
A couple of fixes were applied to clean up event log errors (not clear whether they caused the reboot in the first place).
MalwareBytes and Avast have done scans and come up clean.

If it keeps doing it, the graphics card is my next stop. Just need to find something that can effectively test its memory. I can always pull it and use the on-board graphics for debugging purposes, since it's not like I can replace the memory even if it turns out to be the problem.

The machine is, I think, 8-10 years old. Kinda surprised it's lasted this long. :)

I told you, I seem to have good luck with used machines ^.^;

One of the first things I always do when a system starts showing non-specific errors is disconnect and re-seat everything you can easily get at: RAM, SATA cables, power cables, etc. Contacts can oxidize over time, or work loose through thermal cycling. It's a 5 minute job that MAY save you time and sanity if it works.

If not, then you'll have to do things the hard way. One trick that can help is remove as many components from the system as you can (disconnect unessential devices, remove some RAM, etc.) and see if the problem persists. If it goes away, you can start adding things back until it breaks again.

Intermittent problems are the toughest to fix, but hopefully you'll get lucky before the smoke gets out.

Rochndil, who fixes 'puters in the day job...

We got it fixed, but still, thanks for the advice :)

Could be your power supply may be dying on you. I had mentioned my computer would shut down whenever I tried starting it up to the computer sales person I know at Best Buy. But as my computer is about ten years old and runs on Windows XP I'm looking into buying a new one. So you might want to have your CPU checked out to find the problem, okay?

Thanks for the advice :)