There may be times that you do not wish your computer to sleep. The default power settings for Windows are set up so a computer will go to sleep after the stipulated minutes of inactivity, thus saving power.
However, sometimes a computer may be busy even though someone isn’t actively using the mouse and keyboard. In these cases, you may not want the computer to sleep.
In fact, most media players and disc burners tell Windows not to go to sleep while they’re running.
But for those times when the computer is busy doing something and the relevant program doesn’t suppress the default sleep behavior, you might to consider using Insomnia from Microsoft.
Insomnia is a simple WPF portable application that calls the SetThreadExecutionState API to disable sleep mode for as long as it’s running.
Check it out at MSDN Blogs | Download.
Also check out Mouse Jiggler & Sleep Preventer.