Why randomly hangs the computer?

I have something similar with a computer.
just noticed the feature, I just have code sitting in the Internet, watch video music of IDT. or simple on the go sites. (on games as not affected)
- that only did not do. and went through the computer and taken 4 times m to the masters. the problem was 6 years in a row. "all this time the Windows were one and the same" - and then recently "it sounds stupid but all are" changed in Windows... and seems to have solved the problem..
I 100 times advised friends to change the Windows , I refused as I had tekik that type of what is Windows, and if some sort of problem with the computer to change Windows is not an option, clearly the problem is the viruses and so on. and I was too lazy..

well, I helped this. but who knows, maybe went through the last time, changed thermocouple on the bridge and video card and stuff like that can also affected.

to disassemble the computer (and PSU does not hurt), clean of dust, cleaned all the connectors - to assemble.
look at the electrolytes.
the feeling ncontact somewhere or oxide.

Classic blade (falling contacts from overheating) memory chips or part of the processor on the graphics card. Warming up in the oven can return the contacts to normal, and perhaps will work fine for another few months or years. Well, or just to take in service for reballing the memory chips and the graphics processor.