It always seems that whenever someone asks me to look at a computer its usually a complete mess inside as far as dust and pet hair, A clogged up heat sink can fry your system quick and it also makes cool popping sounds.
Needless to say if money is no object to you then don't worry about cleaning out your computer however if you are frugal like me then you should try and clean your computer out once a month or so. The compressed air works great or a shop vac with the hose reversed to blow air (be mindful not to get to close) this one was just dropped off to me this evening but i believe its got more serious problems than some dirt.