Compaq is known for some systems where large heatsinks were used on the P2/P3's, and only used the air flow from the power supply go over the HS, and thus, didn't use a fan (quieter/cheaper that way). You can't get away with that with athlons, nor durons.
Cyrix III's on the other hand (the Samuel version)....those don't require a fan, though they still output over 10 watts of heat, and therefore, still get awful toasty without a fan.