Verifique se esta configuração está ativada na sua BIOS e desative-a.
Extraído de um manual online do Asus P4XP-X
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AC PWR Loss Restart [Disabled]This allows you to set whether or not to reboot the system after powerinterruptions. [Disabled] leaves your system off while [Enabled] reboots thesystem. [Previous State] sets the system back to the state it was before the powerinterruption. Configuration options: [Disabled] [Enabled] [Previous State]
A propósito, há também uma opção de estado de energia
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Automatic Power Up [Disabled]This allows an unattended or automatic system power up. You may configure yoursystem to power up at a certain time of the day by selecting [Everyday] or at acertain time and day by selecting [By Date]. Configuration options: [Disabled][Everyday] [By Date]
Veja se esta opção está desativada