OK, there is a way to build a noise free PC, but you probably won't like it
What you do is this - buy a Coppermine P3 1000MHz with a 133MHz bus (very important!).
Then, put it on a 815 or 810 motherboard, and clock the bus down to 66MHz. It will run at 500MHz, but won't even need a fan (just a heatsink). Keep an eye on your temperatures, mind.
~ I know the human being and fish can coexist peacefully ~