BleepingComputer.com: PC Shuts off while gaming - CPU overheating?

Long story short, My PC shuts off when playing almost any game. The more graphically intensive the game is, the sooner it happens. Before I can start it back up, I have to wait at least a few seconds, unplug the power cord, and plug it back in. Obviously this seems to be a heat issue, but I'm not sure if it's the CPU or what. It feels pretty hot to the touch after it shuts down. I've had the system for a little over a year and never had any problems until now.

Here is the system: http://www.newegg.com/Product/Product.aspx?Item=N82E16883229137

I should mention that up until a few weeks ago when it started happening, it sat mostly unused for about 3 months, because I just did not have any free time to play. Circumstantially, it seems like that has something to do with it, but I don't know what. I was going to just buy a new CPU heatsink/fan (stock fan sounds like a 757 taking off anyway), but I thought I would ask first to make sure it is what I am suspecting that it is. Is there a way I can diagnose it myself, and if it is the CPU, is buying a new heatsink sufficient? What should I do if it's the GPU?

Thanks in advance.

Did it come with any temperature monitoring software? many systems like this do. If not, download and run HWMonitor And see where your temps are at.

Good idea, I hadn't thought of that. I will check, and if it didn't come with anything I'll try HWMonitor.