Windows 7 Randomly Puts Itself to Sleep/Powers Down

Hello,

I don't know exactly how to word the issue I'm having, so I'll just describe it. I just built a new computer, installed Windows 7 and all that, worked great. However, recently, when I leave the computer on by itself, after a while I will come back and the computer will have randomly shut itself off. Mind you, it isn't a full shut down, such that when I hit the power button again, it doesn't go through a full windows boot, just takes me to the login screen to my user account.

What also happens at this time, is that my computer does not recognize my monitor's optimal resolution. I then have to go into the control panel and hit detect every time this happens, so that the computer displays on the whole screen.

Is there any way to fix this?