It sounds an awful lot like an overheating problem. Unplug the computer from the wall and open the case. Carefully using a can of air, clean out the built up dirt from inside the case, and from the fins on the CPU heatsink.
Try this first and see if it solves your problem.
I've read other posts, and have not come to a conclusion.
I have a Compaq Presario, running widows XP.
I had used the computer Tuesday morning with no problems. My daughter used the computer and all of a sudden it just restarted, she said. By the time I got home at 2, she said it happened a few times.
We did the round robin to try to find out what happened. Of course she said , Nothing!, I was only on myspace. Both my husband and I said after looking at her, said you must have clicked on something. No, I swear she said.
I checked the history on all the log ins. Nothing unusual. I did a system restore. Ran Adaware and Spybot. Tried to run the AVG and it decided to restart.
Did a disk cleanup and then ran the AVG and this time it finshed. It found nothing. That was done running Wednesday morning. It still seemed weird though, so, we disconnected the cable, and left the computer off all day.
Get home from work, turn it on, plug cable back in. Everything seems fine till this morning. Monitor goes black. No signal pops up. Green light still on, on the tower. Still green a minute later. Turn on and off the monitor. Still black and light still green. Manually turn tower off. Wait 30 seconds. Turn back on. Happens again. Turn it off again. Wait. Turn on, run Adaware again.
Have to come to work, so I can't proceed any further.
So, if anyone has any clue as to why these things are happening, I'd appreciate any help.