I have a five years old Dell laptop that has been working fine until a month ago. I had Youtube on for couple hours and my wife got mad. She pounded on the left corner of the laptop, right under the keyboard. The screen started to flicker and the laptop just shut off. I was able to format the hard drive and re-installed Windows XP after several attempts. Now the laptop keeps shutting itself off. I am thinking about replacing the hard drive, but I'm not sure if that will solve the problem. Does anyone know which componet is causing the laptop to shut itself off?

You can just remove the Hard Drive and see if it shuts off by itself or not
If not, it may just be a Windows issue

Here's a couple of helpful threads:

Tutorial: No POST (Power On Self Test)
UBCD4Win (backup and re-install Windows clean)
Dell Drivers and info