Look around in your BIOS. There must be a setting somewhere that tells the computer to power on automatically when the power comes back on after an electrical shortage. Tell your computer to stay off in this situation. It's a useful option when you use your computer as a server though it seems to be more of a hassle to you.

I cannot tell you for sure how to get into your BIOS because it differs between manufacturers. Most of the time, you have to hit F2 at the boot screen. If it doesn't work, look carefully at the boot screen. It should be written somewhere how to get into the BIOS. Be VERY careful in there cause you could get into more trouble than you are already in if you play around with the wrong settings. May I suggest that you write on a paper any modification you make just in case you would want to go back.

I'm sorry I cannot be more precise, but like I said, all this differs between manufacturers.

Good luck !

3rd Attempt (Success!!)

Problem recognized: Wake on LAN

Right click "My Computer"
Select "Properties"
Go to "Hardware Device"
Click "Device Manager"
Select "Network Adapters"
Select "Marvell Yukon .....PCI-Fast Ethernet Controller--> right click ---properties--->power management-->deselect "Allow the computer to turn off this device to save power"--->Go to "Advanced" tab--->put value off for "Wake from shut down" --->value none for "Wake up capabilities"
press OK


If this doesn't work then check your BIOS

Restart ---> press F2---> go to "Main" tab --->Network boot [Disable] --- Save