why does my desktop just shut off?

I have a Compaq 5310US desktop that I’ve been using for four years and it has worked well. Earlier this year, I upgraded the RAM…that’s the only change I’ve made to the PC. However for the last few months it just started shutting off…not shutting down but shutting completely OFF!

I use it mostly just to surf the net and MS Office documents but it’s really annoying to be reading an article online or discussion…or just checking FB or email and have the machine just shut off.

Any ideas as to what may be going on? Power supply issue? Spyware/Malware/Virus? I mean really? It’ll sit there on the desktop for hours and hours but after a while of being on-line or writing a document…all of a sudden, it just shuts completely OFF!

Well?