there are TWO areas that I would check,,
lets not focus on the rating of the Mobo and the Power supply right now,,
Major has mentioned one area to check easily,,, in the control panel check the Power settings,,, make sure that it is set to "shut Off"
Second spot,,
in the BIOS,,(when you boot the PC,,and it says "Press del/f10 or what ever to enter setup) Once in the BIOS look for the Advanced Power settings... make the appropriate changes from the options provided
cheers