Opps, where is my XP’s StandBy Button??!

Opps, where is my standBy Button??!!!!
Hii, I have a problem with my OS which is WinXP Pro., the StandBy button is un-clickable or unenabled, I have just two options in the power; to restart my Computer or to shut it down and StandBy is unavailable,why? I dont know…. please I need your help….