There is a setting in the BIOS about what you want your computer to do once it loses power. It can restart or stay off. To get into the bios, the PC will say "Press Del to enter BIOS" or something to that effect. Sometimes its Del, or F2, or F10, it depends. Then go through the menus and look for power settings, and something like "On Power Off..." Change it to Stay Off.
The menus are different depending on who made the BIOS, but they are usually similar.
Good luck
Daniel Zee, June 2007