The same reason you can't watch a movie is why you can't complete a scan. It is overheating and shutting down or crashing. A scan is very CPU and hard disk read/write intensive. Pretty much the same thing when playing an HD movie. If you are certain it is overheating and not a hard drive issue which can have the same symptoms, then you will probably need to repaste the heatsink on the CPU since it sounds like it is not seated properly. You will need to google the procedure since all laptops are different.