Try this, Go to Start\Run type MSCONFIG press enter go to Startup tab at the top right, check what programs that do not need to start at Boot by removing a tick in box it stops it loading at start. You need to leave Anti-Virus and Firewall running. You can put the tick back if you find you need it. When computer starts a box will pop up, put a tick in the little box to confirm it's OK, see how it goes but it may take some time to find what is doing it
sounds like possible corrupt filed if you have the disk click start/run ans type in sfc /scannow it will ask for the disk, if you have not got it might you be able to system restore to before probs started? can you recall what you were doing when this first happened?
Well the above reason could Be heat related or a Driver conflict or that needs updating. Is the CPU fan working as it should? you need the side of the box to see. And if this is not a new comp may need a can of air to clean it out
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