Desktop

First check the hardware to see if the power supply and cpu are operating properly.
The other option would be to check for any virus,also run scandisk to check the hard drive is not gooing out

we had a couple of pcs doing that.
for one pc, it was a hardware problem and we had to replace the m/b.
for the other pc, after finding no apparent h/w prob, we wiped the harddrive, fdisk, reformat, reload all the data, and it stopped.

We had a pc that did the same thing it was a chassis fan causing a short. Once we replaced the fan the system ran fine.

As per the A+ Certification guide: "A computer that reboots occasionally probably has a faulty power supply".

I've seen that more than once myself too.

I see lots of good answers but I had a system with this problem and it was the cpu fan quit working and when the cpu got hot enough it would shut down the system.