Vista - Windows shuts down when im away from computer!

I don't know what the deal is, its definately not a hardware issue because i never had any problems whatsoever with xp pro x64. the problem is my computer just shuts down apparently when im away from it for any length of time, i set my hard drive to defrag and i come back a few hours later and its turned off, and when i push the power switch the computer doesnt post, so i have to turn the power switch off on the power supply to reset it and it turns on fine, and when it starts back up... its back to the defrag screen.... only seems to happen when im away from the computer, is it failing to go into sleep?