I suppose the question is have you just noticed it?
or has mrs squad re-modelled the layout
if yes ... it's probably muck
PC and laptops are just clever hoovers ... so anything that's in the air is sucked in ... and then finds it easier to stay than get out.
if you're feeling scientific - download and install
http://www.cpuid.com/pcwizard.php
it will show some interesting (?) info
if you're not - just open the box and look
androcles has covered the basics
laptop - narrow vacuum noz over the vents
desktop - vac the psu - and external vents
but then brush the rest out (don't disturb the heatsink or you're stuffed)
then vac the cr@p out of the bottom of the case