CPU HSF: Central Processing Unit's HeatSink and Fan (might also be HeatSink Fan).
Not too sure whats the difference between BTX and ATX, but being BTX definately doesn't mean you got 2 fans....
Cleaning the fans will help, but cleaning the heatsinks is the main thing. This may not be easy, since the fins are pretty small and may be hard to stink a piece of cloth in or something to clean it.
A can or air usually works better (I use it all the time). Get a can of air from your local electronics shop and spray onto the heatink and watch the dust fly out. Use it on your fans too. Wiping them clean yourself may disalign the fan (happened once to me, the blades are disaligned and keep hitting another part of the fan, causing great audio discomfort). Being gentle helps, but if you're cleaning 6-7 of them....
To apply thermal paste:
http://www.arcticsilver.com/arctic_silver_instructions.htm