Several people had told me it was probably a virus, but I keep my virus definitions up to date and run frequent scans. And, I didn't get any of the warnings of shutdown I had heard about - no warning, just blinked off and rebooted. I sent the tower to my nephew, who checked it out, and replaced the mother board. Problem solved.
I was adding memory .. after I put the chip in it wouldn't even turn on. I fiddled with it .. because my son kept saying my computer probably wouldn't recognize it. I changed the configurations several times (old motherboard). I never could get it to work. Finally, took the new memory out, computer came on and proceeded to continue to restart. It was almost funny. I knew it wasn't good for the computer so I shut it down.
Fortunately, my son has his own business upgrading computers and all I had to do was order the parts and he put it together. As a result, I upgraded everything, but I suppose in a couple of years I'll have to do this all over again ... LOL!!