If you have a Shop Vac, connect the Hose to the output (be sure Shop Vac
is empty and clean) then take it and your Tower outside and blow it out. Be
sure to get the Fan Grill and any heat sinks on the board. Usually Dust is
NON-CONDUCTIVE but animal hair etc may get on the board and short
low-voltage items. Blow out the keyboard and power supply. Be careful as
not to blow too hard on components where as the Laser lens (acrylic) may blow
into another Time Zone.
I do mine every six months. it is amazing how much stuff gets sucked in. I
have dual fan power supply and it gets nasty in there rather fast.