I think it just turn off the pc, instead of running the shutdown. Problems like these are caused by:
1. Dodgy PSU's, They cannot provide enough power to keep things running. Check the 3.3 5.0 12.0 Volt rail values, they should be in a 5% range:
3.3V = 3.15V ~ 3.45V
5.0V = 4.75V ~ 5.25V
12.0V = 11.4V ~ 12.6V
same goes for the negative values (although they are allowed to be in a 10% range).
Many people say 3.3V should be 3.3V, but there's no magical PSU that can deliver perfect voltages.
2. Heat, check his airflow. Put unused PSU cables away from the fans. (If he has any). His cpu should never go above 55 Degrees.
If there are no fans, place some (Place a fan at the front first, later you can add one at the back.)
3. Loose cables. Most of the time, when a cable is loose the PSU (in my case) won't even supply power (prevents fires, etc).
Follow all used cables, disconnect them from the device they are connected to, and connect them again.
I had this problem a few days ago, found out that the one that goes to my motherboard was a bit loose.
4. IDE Devices. If a harddisk other device is failing, windows will respond with a reboot or shutdown. Most of the time, when it's a harddisk, you can hear a sound coming from it "tick!".
If all above doesn't work, I cannot help you further. (to fix something, I have to do it myself
)
goodluck
