There are quite a few "small format" computers that use passive cooling measures and have no fans for air extraction etc.,
However with an ordinary home computer fans are usually a necessity.
as well as the usual fans on the PSU and the cpu/heatsink its usual to have fans to keep the general insides of the computer and the electronics at a reasonable temperature.
Remember as our own Digerati says " heat is the enemy of electronics" I tried to find the excellent article he wrote on the subject but alas I could not find it!
So here's a link to another article that gives you the details:
http://www.tomshardw...puter-cool.htmlRegards
paws