Sleep & Auto Shut Down

Just purchased new computer with 7 home edition preinstalled. It shut down on it's own overnight while I was downloading? How do I stop it from doing this again.

Remember, Google is your friend. Windows 7 sleep mode settings. I would start here.

I would also caution you that unattended downloads is probably not a wise move. It is important to note that a favorite distribution method badguys use to distribute malware is through torrents and P2P filesharing, especially of copyrighted materials.