How to activate PC Sleep mode in Windows Vista?

Click the Power button

Under Windows Vista, import a new power mode called Sleep mode
The Sleep mode has similar features as the Standby mode, including data-protecting in the pause status and low-power consumption.
If the Sleep mode is selected, it could take a few seconds to restart Windows.

By factory default, clicking the Start button and the Power button can activate the Sleep mode
 

When clicking the Power button, if you can not convert to Sleep mode or you want to change the specific action after clicking the Power  button, please take the following steps: 
 
 Please refer to the FAQ: How to change the action after clicking Windows Vista Power button?