Windows Vista imports a new power mode called Sleep mode. Sleep mode has similar features as the Standby mode, including data-protecting in the pause status and low-power consumption. If Sleep mode is selected, it could take a few seconds to restart Windows.
By factory default, clicking the Start button and the Power button to activate the Sleep mode
Please see this FAQ is Sleep mode is not activated after clicking the Power button, or to change the specific action after clicking the Power button - How to change the action after clicking Windows Vista Power button?