Why do my computer keeps restarting at startup?

Help!! My computer keeps restarting, i've tried everything!!!?

• Here is the thing, my computer keeps restarting every day when I turn it on for the first time in the day, it works fine for 5 minuts, then suddenly shuts off and restart( sometimes restarts again after 5min). After my computer works fine for 2-3 hours before turning off again. I have already looked on the web for valid answers and I’ve tried everything, I’ve tried disk checking, cleaning, defrag optimize my pc, cleaning the dust on the computer fan and on my cooler, I’ve checked all my devices worked fine on device manager. I even unticked the box stating “automatically restarts” in computer- properties- advanced system settings-startup and recovery. I’ve tried to use my computer only on battery or only with the power plug, yet it stills restarts. I’ve also tried downloading the latest update for windows vista and had no luck. I've completed every clening and optimize option with advanced system care and AVG antivirus as well as most option on Ccleaner yet nothing as changed ( aside from my computer getting faster) Note: I have an acer aspire 6920G with windows 7. when my computer restarts, it does so, not peacefully as when a new update has finished downloading instead it randomly turns off ( I can hear a loud click as well as the fan stopping). Any help would be greatly appreciated :)

• Answer:

  Your computer may be over-heating, a lot of computers do this, my sister has the same computer and this was the problem that she had to face as well - it always turned off after around 3 hours and we later found out it was because it over-heated and turned itself off. I have no idea if this is what's wrong with yours, but it sounds just like the problem my sister had!