1. ..Opened the Control Panel, double click on the "Display" icon, then the "Screen Saver" tab, then the "Power" button, then the "Advanced" tab, then change the "When I press the power button on my computer" selection to "Shut Down", then click on "apply", then OK.
2. RIGHT click on "My Computer", choose "Properties", then click on the "Advanced" tab, then click on the "Settings' in the "Startup and Recovery" section. When that loads, UNCHECK the box next to "Automatically Restart" in the "System Failure" section, then click on "OK, Apply, OK, etc.
3. What happens when you hold the power button in for 20 seconds? Does it shut down then or does it still restart?
4. Replaced The Power Supply? I've had power supply units that simply wouldn't shut off correctly..
Hope this helps.
Grif