check the heatsink on the cpu isn`t loaded up and choked with fluff (`specialy between the vanes, a bit of card pushed through each slot will push out the muck), same for the fan blades (fluffy blades stir air, they don`t move it at speed), for some reason the cooling system on a PC will attract dust like anything, I normaly clean mine out every 6 months rather than wait for overheating to cause errors, if you have it down at floor level then thats a lot worse too, they act like vaccum cleaners and seem to suck up every little bit of dirt and dust your feet kick up and use it to choke up the heatsink.
if you listen you can hear the inter-dimensional zombies calling.....branes, branes......