Good.   Now go down to the nearest mass market retailer (office supplies or
electronics) and buy yourself a can of compressed air.   Remove the side panel
from the case, take the rest outside, and blast compressed air into the back
where the fan intake is and into the other openings of the power supply. You'll
probably be amazed at the accumulated dirt and dust that flies out. Don't inhale
the stuff.   While at it, use q-tips to dislodge the dirt in the CPU fan and
heat sink and give it some good blasts of air, too.

This is or should be normal maintenance on a computer, maybe once a year, more
often if used in a dusty environment.   I did this with 7 computers a couple of
weeks ago, and they had not been cleaned out in 4 or 5 years since being
installed initially.   The owners were lucky that they all had run as long as
they did, although another even older computer croaked the previous week, which
is why I did the cleaning that I did... Ben Myers