In Windows Vista the power and sleep button behaviour can be changed. The default behaviour is when you press power button the computer shutdowns and when you press sleep the computer goes to sleep.
This default settings can be changed through the Power and Sleep button settings and this can be accessed through Control Panel -> Hardware and Sound -> Power Options -> Choose what Power buttons do
When you press the power button you can set it to Do Nothing or Sleep or Shutdown
When you press the sleep button you can set it to Sleep or Do Nothing