Laptop Spontaneously Shuts Down

[signal]

People are constantly asking me to fix their computers. So someone has given me a new project. I am at a crossroads with this one so I am now seeking 2+2 comp/tech advice.

Problem: The laptop spontaneously will lose power and turn off.

I had the girl backup all her data before I did anything. She then gave it back. My first inquiry was to diagnose whether the problem was software or hardware based. I reformatted the hard drive to provide a clean install, thinking perhaps she picked up a virus or something. As I was installing the AV the laptop then spontaneously shut off. This has occurred a few times now. The problem has not been fixed, but now I know it is most likely hardware related.

When this sort of thing happens to a desktop PC, I just replace the power supply. But, this is a laptop and I think I am out of my element with this one. So advice please: can I fix this? Open up the laptop & replace the power supply? Or do I tell the girl to send it back for factory repair?

By the way it's an Inspiron 2200 from Dell running Windows XP.

Any advice would be appreciated. Thanks.