Why does it keep restarting?

Two of the systems I do trouble shooting on both have the same problem. When shutting down instead of the you can turn the power off screen comming up, the system reboots. One of the systems is an E-machine, the other is a gateway. The E-machine hasto be unpluged or it keeps trying to reboot. The gateway can be shut down. If you use the power button. As it is doing the bios tests before another screens appear. Help it is driving both me and the owners of the systems nuts!