Oh sure, I let my MaximumPC magazine subscription lapse and they put out this gem: How To: Build a really, really, really silent PC "Is this thing on? We assembled a Pentium M-based rig with a fanless power supply, and ended up with the quietest air-cooled PC we’ve ever heard."

It's not a cheap proposition though using a nearly 500 dollar pentium M CPU

Ingredients:
Intel 2GHz Pentium M 755 ($440, www.newegg.com)
AOpen i855GMEm-LFS motherboard ($275, www.newegg.com)
Zalman VF700-AlCu VGA cooler ($40, www.zalmnusa.com)
Antec TriCool fan ($20, www.antec.com)
Antec 350 watt Phantom ($175, www.antec.com)
1GB of Corsair Micro DDR400 ($200, www.corsairmicro.com)
Pioneer DVR-A08XLA ($150, www.pioneerelectronics.com)
Smart Drive 2002 hard drive enclosure ($55, www.endpcnoise.com)
Antec SLK3700-BQE ($100, www.antec.com)
Floppy drive ($10, www.newegg.com)
Windows XP Professional ($200, www.microsoft.com)
Arctic Silver 5 ($8, www.arcticsilver.com)
Akasa Paxmate ($20, www.akasa.com.tw)

UPDATE kdhamann rightly points out that SilentPC Review is another really good resource for info, and well... reviews of quiet PC and noise dampening components.