PC noise is down to case design. And that's down to cost, really.
I'm typing this on one of my "found" PCs, dual core Athlon XP 64 running Windows XP. It's a fancy looking unbranded case, probably designed for a small business server. Quite a lot of fans in it, and filters too to stop dirt getting sucked in.
BIOS settings have options to adjust speed of fans based on measured temperatures.
I've tweaked this as optimally as I can, as a result, it's lovely and quiet as I power it up, but the fans gradually get faster as it warms up. It has an almost revving-engine effect, as when you're hammering it hard (video conversion, driving around in Google Street view), the fans speed up noisily as the processor gets worked hard. All good fun
That said, I've also got an HP PC here that is incredibly quiet. Not silent, but very quiet. It has fans, again temp controlled, but the case design is such that efficient airflow is achieved quietly.
A bit of googling will find you "silent" power supplies, CPU coolers etc for PCs, if you care to pay the premium.
Ian