Help - my lapto is not behaving

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For the last couple of weeks my Dell laptop, which is more than 3 years old, is not doing well. If I run a antivirus scan, any software that takes a lot of CPU usage or even have too many browsers open, the CPU usage keeps at 100% for a few minutes and then the laptop abruptly turns off. When I try to restart it immediately, it comes on and then turns off again. I have to wait for about 15 min before I can turn it on again. This is driving me crazy. Any suggestions?