My computer is shutting down and freezing. Not viral, and system restore and goback have been used. Please hel

My computer is not starting, it will freeze or just shut off entirely before I can act. I have tried cleaning, my computer had quite a bit of dust. It worked, and then happened again. I need some help cleaning specific parts, where should I focus and is canned air enough? I also can't seem to open my internal fan, no screws or anything and pulling hard doesn't work. Lastly, I receive an "overclocking error" which I guess is from the person who put my comp. together, is this something I can remedy? Thank you so much, this is dire for me.