Latitude d400 suddenly starts turning itself off

this is so frustrating. it usually happens when I am surfing on the net. sometimes I can't get the computer to turn on when I hit the power button. other times it will respond to the power button and then turns itself off after I sign in, or after it re-boots, or when it comes to the screensaver page. and it gets worse each time I try to power up again. I saw online that it might be a fan or a bad motherboard, neither of which do I know how to fix. I'm unemployed so doing it as cheaply as possible is important. Thanks!