Having just had to replace the power supply in my p.c I was amazed just how much dust and fluff there was inside the unit. If your p.c is standing on the floor the fans suck in all the muck and this stops the thing from running cool. Worth giving it a hover out every now and then.
You shouldn't use a hoover inside a pc as they can generated electrostatic discharge that can knacker your pc. Best to blow dust out whilst holding the hoover nozzle in the other hand to suck up the airborne dust.