You can send the VK_OFF key to the system, which is more or less what
happens when the user presses the Off button. Unfortunately, you weren't
specific about *what* device, so I can't tell you whether this will work or
not...
The two methods shown in the article referenced by Neelima should both work.
You'll have to learn, at the very least, how to call C/C++ functions from C#
or VB.NET; you just can't live without that capability...