shut down?

You wouldn't happen to have an ATI graphics card by any chance?

I had a similar problem whereby the fans on the card would kick in for a few minutes then my pc would shut down into hibernate.

To fix the problem make sure that all the fan blades are clean, all the vents on the PC case are free from fluff and dust and if required fit an exhaust blower in the slot next to your graphics card to help the fan get rid of the excess heat.

I did all this and the problem has never appeared since.

Basscone
November 2005