Computer shuts down

Hi:
I work were this a network station. Some of the computers OS WinXP and Win2000 automatically shut down. It gives no error message nor nothing. It has no virus. I have checked that and also the temperature stays about 56 Celcius. It happens around twice a week. And only affects 3 computers out of many.
The computers are 1.8 and the other one is 2.5.

If someone can give me any clues on what may be happening I would appreciate it very much.

Thanks

56 Celcius is pretty much.
My computer has a self shutdown when it reaches 60 celcius.
If it's 56 at normal, then when you start working with it, I pretty sure it will go over 60.
My suggestions would be checking if the fan is installed correctly and if it is, buy a new fan.
They aren't that expensive.

Where are these computers located and at what time of the day do they shut down? Do they get back up by themselves?

It might just be temperature... are these Intel or AMD CPUs?