HELP. My computer keeps on shutting down by itself!

I have a Dell Inspirion 1150 for months now. For a month ago, it keeps on shutting down by itself and I dunno why. Is it the CPU, RAM or something else? It only happens when I am using it for encoding a movie, converting a large files, etc. It doesnt happen when I am using the basic stuffs, like word processing, web surfing, etc. I am pretty sure that isnt a software matter because I reformatted my computer like couple of days ago. Please help! I need to find the source of this problem so I can try to fix it. TIA

JC