Modern PCs will have at least 2 cooling fans: one for the power supply and one for the CPU. A third cooling fan can sit on the graphics card to cool the graphics processor (e.g. 5VDC at 60 mA=0.3W). An extra cooling cooling fan could be mounted on the motherboard to cool the Northbridge chip (also 0.3W). Naturally, PCs and servers with more than one processor need a cooling fan for each processor. And if all this doesn't help, one or more case fans (10-12 W) can be added to ventilate the entire casing.
Before the 486 processor, the power supply fan (up to 4-5 W, RPM controlled internally or by Southbridge chip on the motherboard) and passive heat sinks (finned blocks of aluminium or copper near the CPU and other hotspots) used to be enough for cooling. Now, active CPU cooling has (almost) become a necessity.
For Pentium III and comparable, the CPU cooler uses some 1,2 to 1,5 W (12VDC at 85-130 mA) to keep the CPU at lower than 40 ºC. For P4 (Thermal Design Power at more than 70 W with 180 nm technology) fans at 3,6 W as maximum (12VDC at 300 mA) exist. Note that these are maximum power requirements: the CPU cooler fan speed are controlled on the basis of actual temperature (control by South Bridge chip on the motherboard), so the actual power will be a fraction of the rated power.
Alternative CPU cooling methods are heat pipes (usually combined with natural convection in an aluminium casing and/or with a fan blowing at the hot end of the heat pipe), water cooling (this system employs 2 heat exchangers at either end, a circulation pump and a fan placed outside the PC), a 'refrigerator' (with compressor, evaporator, HC or HCFC fluid, etc.) or a Peltier cooler. These refrigerator solutions are rare and used mostly with overclocked CPUs. Depending on the heat that has to be removed, a refrigerator may use up to 200W (Windows desktop) or 300 W. A water cooler might easily take some 20-40 W and of course the heat pipe solution with natural convection doesn't use any energy at all. The reason why designers choose e.g. a heat pipe has nothing to do with energy (yet) but is mostly instilled by the desire for an absolutely silent machine.
Courtesy of the French Energy Agency ADEME, Future Electronics project. -Expanded by VHK.