Why do my computer keeps restarting at startup?

Help needed! My computer keeps restarting ( even after restore to Factory setting)?

• Okay here is the thing, my computer randomly restarts after 5mins everytime I turn it on for the first time in 4-5 hours, then it works fine for 2-3hours before restarting again. I have looked on the web for answers and have done everything recomended including: - disk checking, cleaning, defrag, optimize -cleaning the dust off my CPU fan and cooler ( my computer is not overheating when it randomly restarts. - i've checked that all my devices were working fine on device manager -i've unticked the box "automatically restarts"in computer-properties-advanced settings-startup and recovery. - i've used my computer on batteries, on just a plug, on and without an internet browser turned on, yet it stills restarts. - i've downloaded all the latest update for windows vista - i have also recently restored my computer to Factory Default, the problem still persist but it seems to occur less often at least. as for the registry cleaning and optimizing i've done it with several free programs but every time it tells me i need to buy the full version to fix all the "errors". Does anyone a good free program for registry cleaning? note: I have an Acer Aspire 6920 with windows vista + when my computer restarts it doesn't does it slowly like an automatic restart after an update, but instead i hear a sound of a rather loud "click" with the fan stopping. Any help would be greatly appreciated :)

• Answer:

  if you hear a click and the fan stopping then its your cpu thats overheating and making it restart take it in to get looked at EDIT : is this a laptop or a desktop plus i dont know how its possible to stay on afterwards but i had an old old socket 478 pc that would over heat and restart when ever it was standing up right and then sometimes later on would stay on switched it to lay it down and never overheated again found out the clips holding heat sink on were broken and it only had a partail part touching so didnt take all the heat off , maybe yours is transferring enough heat once it heaps up the heatsink