PC Sleeps For No Apparant Reason

My desktop PC sleeps, sometimes, even though I believe I have set everything in Power Settings to prevent this.
I happens only 1 or twice a week and after about 15 minutes of no activity.
Most of the time I could leave my PC on for several hours with no activity and it won't sleep.
Not sure what sleeps. I could be my monitor.

Any Ideas??
James

You have system specs consisting of "Windows 7 64 bit" so I'm a bit confused.

I updated my profile.
Windows 8.1 64 bit.

Try posting a Speccy published url for your system. The guessing game is a time waster. This shows how.

Oh well.