Black Screen

Hi Ron, Thanks for the response.

Never replaced battery (assumed since never lost date on  power outage it wasn't an issue)

Light is white solid, light turns off when it auto restarts, then back on again for 20 minutes ish. The restart every 20 minutes: computer is trying to fix itself?

Can't find anyone with bootable cd/USB (sell them at computer stores / best buy?). 

Win XP disc has no effect

Win 10 ISO: they have 6 choices, which do you recommend?

Thanks!

Replaced the motherboard battery recently?

Ailing power supply?  What color is power button and is it steady or blinking when PC won't boot?

Do you have (or borrow) a bootable CD/DVD or USB stick you can try?

You can download the Win 10 ISO file for free from Microsoft and use their tools to create a bootable USB, if necessary.  

Fan(s) running, no beeps

No caps lock light on keyboard, no mouse light

Hard drive light blips ever 3 seconds (forever)

If PC is set to sync automatically from the web, you might not see any lose of date/time.

Battery is only ~$2 so may be worth replacing, if nothing more than to rule it out.  Reboot and IMMEDIATELY press F2 to open BIOS setup. Win 10 loads very quickly so you have to press F2 quickly or you miss your chance. In BIOS setup, copy down all current settings, just to be safe. After you copied the current settings, leave PC alone without exiting BIOS setup. See if it crashes after "20 minutes ish" or longer.

Now power off, unplug and press/hold power button for ~15 sec.  Reseat RAM modules and all PCI-e cards in their slots and use canned air to blow out all the dust. Install a new CR2032 battery after everything is reseated. Then reboot and see what happens now.

Any signs of overheating? Are all fans working, including CPU fan, video card fan (if any) and power supply fan?  Open Windows Event Viewer and look for errors around the time of the last crash. That may point you in right direction.

Reboot and IMMEDIATELY press F12. Look for option to run Diagnostics, especially CPU fan, RAM and extended hard drive tests. This may take a while so be patient. Copy down error messages, if any.

Not sure what 6 Win 10 options you're seeing, but you can only reinstall the version you have now. So you want 64-bit Win 10, either Home or Pro, and I presume US-English or whatever language is currently installed. www.microsoft.com/.../windows10

I'll get a battery tomorrow.

Screen is always black (monitor & cable are good, confirmed with other computer)

F2 / F12 don't work. The keyboard & mouse have no lights, seem dead (but they work on other computer)

No signs of overheating, vacuumed it, all fans working.

On that Win 10 link: Select Edition: Creator or Anniversary, then there are 3 options of each.

Replace the battery and see if that helps. If not, consider a PSU issue...

You want Creators Update which is the latest update after Anniversary Build.  Not sure what 3 options you see, but you want 64-bit,  Home or Pro, depending on which you have now. You do not want an N version.