Underclocking and undervolting CPUs has been a mainstay of silent computing. AMD's Duron platform was excellent for this. Not suitable for gaming, but fine for

Underclocking and undervolting CPUs has been a mainstay of silent computing. AMD's Duron platform was excellent for this. Not suitable for gaming, but fine for daily computing. Big heat sinks with heat pipes work pretty well. Big, slow fans when there's no way around them. Plastic rivets instead of screws and rubber washers manage to keep vibration noise down (fans, spinning disks).