My iMac (21,5 Inch, Mid-2011) is running uncomfortably hot.

I've been noticing recently that my iMac has been running hot. I bought it used two years ago from a DJ, who only used a few times for video and music storage. When he didn't use it, it just stood there, collecting dust. I have been using it (almost) daily, mostly for music production.

So, my question is; Is there anything that I can do to make it "cool down"? I've been thinking about vacuuming it, but is that really helpful?