power supply??? - DOTNET

I have an win xp I purchased a new computer 2 years ago about 2 months ago I
noticed my power light was not as bright as it should be. Each day the light
go dimmer. Now you can barely see it. I tried logging on which I'm able to
do, but once computer is on for 5 mins, it automatically shuts down, no power
and the only way I can re start is to use the reset button and again it will
only stay on for 5 mins. Is this my power supply? I tried doing an restore
computer but when I try to do so it cuts off. Is there somethings I can do
on my own to make this work again before bringing to be serviced