Why does my CPU fan stop spinning after Windows Vista starts?

It is such a strange behaviour on my Acer Aspire 5720 system running Windows Vista Italian version.

I have been using my laptop for a year and recently it often shuts down immediately without displaying any message. I discovered that the CPU is too hot, the CPU fan is not spinning when Windows starts, but the interesting thing is that when I just turn on the computer with the Power button, the fan spins well, only after 2 seconds, when Windows Vista is launched, it's time for the CPU fan to stop.

I wonder what happened?

This might be a hardware problem.

Enter BIOS setup, wait a minute or two, and see whether the fan keeps spinning. If it doesn't, you probably have a hardware problem.
You can also try booting to a CD (eg, Knoppix) and see whether it stops spinning.

Slaks suggestion is not one that's thrown out into the air. This is a valuable troubleshooting step that will help you to determine whether or not your issue is hardware or software related.

The fan should continue to work while in the BIOS. If it does not, then you know the issue is hardware related. Most likely the motherboard, or thermal paste may need to be replaced.

FYI: Missing or deteriorated thermal paste will cause the CPU to overheat! or in your case motherboard will shut off power to avoid overheating the CPU.

My Dell laptop has the same problem. I have discovered the following reason for fan not working. When a CD is already present in the drive , and switch on my laptop, usually the fan wont work. As soon as i remove the CD our of the drive, fan starts functioning.