This clever little utility written by an MS developer stops your PC from going to sleep. I find it useful during rendering when I usually just walk away from my system and do something else.

http://blogs.msdn.com/b/delay/archive/2011/03/23/what-next-a-dos-version-free-tool-and-source-code-to-temporarily-prevent-a-computer-from-entering-sleep-mode-32-bit-and-64-bit-versions-now-support-windows-xp.aspx