Laptop shuts off, and wont turn back on.

Ok here's whats going on. I'm working with an HP nc8000. My computer a couple weeks ago decided to just start shutting off on me. The thing is, just the screen goes blank and I can hear the fan and the hd still engaged. I will have to manually turn it off, and if I would try to turn it back on right away instead of the charging light being green like usual, it would be orange and wait a couple seconds and just turn back off again, this time shutting everything off.

This happens regardless of ac power or only battery power.

I know its not a hd problem because I put a spare hd in it and it does the same thing.

I'm thinking it is probably a heat issue whether it be the fan clogged or needing a new heat sink, something along those lines.

If I let it go for about a day and turn it back on it will boot up to windows (XP Pro sv. pack 2) with nothing lost, and i can do what I want for about 5 minutes and then it starts all over, shut off the screen, hd and fan seem to still be running and I have to manually shut it off again.

If theres anyway you can help me, I need this laptop to work, and that would be great.

Thanks,

Jon