Get a better processor heatsink with a quieter fan.
I've got a P4 2.4GHz processor. I changed the heatsink for a (not cheap) Thermalright model, with heat pipes. It can keep the processor with spec in the summer in Spain without a fan at all. Of course I do use a fan - a 90cm very slow and virtually silent one.
Also - check out carefully which fan it is - CPU, case or power supply. You can get all items with extra quiet fans - you simply pay a bit more for better quality.
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