http://www.hushtechnologies.net/ shows a reasonably fast (933 MHz, up to 1 GB of RAM) reasonably cheap (under $1000) WinXP machine that is cooled via heat sinks rather than fans. Another very quiet PC option is the Gateway Profile, which looks like half of a laptop computer mounted on a small pedestal. My friend Doug and I removed the (pretty quiet) fans from a couple of old ones (500 MHz Celerons) and they continued to run just fine.
I still think these guys were the coolest silent PC. Too bad the high price killed it and it’s no longer being updated…
I love the case designs on these systems but often the horsepower is just seriously lacking. A friend of mine made a totally silent computer (though the HD is audible when the room is completely, utterly silent as a very very faint hum) running with an Athlon 1.4Ghz at 800Mhz running at the lowest voltage he could get it to. The system has no fans and is really stable. It runs hot…but thats to be expected. A P4 based system would be even more effective since it would throttle the CPU instead of just shutting down/crashing when it overheats
The Macintosh Cube had no fan and cooled by induction, with a central “chimney.”