My computer freezes when i leave it on to download somthing

[Alpha-G]

Almost every time i leave my computer to download something I come back an hour later to find it frozen and nothing works not even the keyboard or mouse until I restart it. I heard that theres a Microsoft toolkit that can watch PC performance and analyze it so I can find out the reason why the computer stops working ? wish I someone can tell me how to get it and how to use it to find out the reason behind this problem.

Windows 8.1 Pro
64-bit
Processor Intel Core2 Quad CPU Q9550 2.83 GHz
Memory 4 GB

 

[Stirb6]

Its probably due to having 8.1 pro on a core2 quad