Check your furnace filters too,
my friends moved into a town home,
and were all 3 getting sick including their baby.
Found an inch thick dust covering their filter,
and their humidifier was a mess too.
(my old house found like 2 inches of Dog hair).

Time to bump this to the top.
Did some spring cleaning on my i7 950 and went from 74C to 66C.
Don't forget to use a cotton swab on the leading edge of your fan blades.
I noticed the canned air was not removing it!

Thats a good idea--never thought of it--i had a quad core Q6600 that was blue screen very often i finally pulled the cover and guess what a pound of dust on the heatsink--has been running great ever since!!

Seeing this thread, I finally got around to doing that long-postponed cleaning. As expected, there were a lot of dust/dirt everywhere inside my machines. I did the cleaning the old-fashined way: opened the case, dismantled everything, cleaned/washed the parts, and re-assembled everything back. Result: temps were near their brand-new level. The CPU is of course, the darling of the show, and I got a 74°C-to-60°C drop there. Nice indeed .
;

FYI for those using compressed or otherwise cans of air make sure you don't use one with a Bitterants. A lot of compressed air has Bitterants in it which corrode the pc components over time. Bitterants are substances put in certain products to try to prevent dummies from huffing the product since it leaves a bitter odor in their nostrils if they try it.

As for vacuums make sure you use a vacuum made for cleaning electronics. If you do not have one read http://www.ehow.com/how_5568198_vacuum-computer.html

You could damage the computer otherwise.

A number of computer gaming cases have dust screens for the fans and some have a detachable filter for the power supply fan since it is bottom mounted which means the heat doesn't go onto the motherboard and other innards such as video cards. These filters are a big help. I have often had to wipe the filters after just a few days.