All of my computers run 24/7, including the Mac Powerbook 17, which might be shut down twice as much as the 2 Win2k desktop 'puters. It does stay folded a lot, to preserve the display.
The Powerbook is the baby of the bunch at just under four years old. My Win2k computer was invoiced for shipment on 6/6/01, and my wife's computer invoice reads 10/10/98. Many upgrades and repair/replacements, but they still have the same motherboards and processors that were shipped on those dates. The hard drives are very different, but they were only changed in the past year. Hers has one more stick of RAM and an OS that are not the same as they were back then, but mine is still like it was, except for all the USB connectioned peripherals.
Booting is pretty tough on a computer, from everything I hear, so I do that only for health reasons... maybe twice a week, unless a storm shows up, and we get some doozie storms around here. UPS's on all desktops, by the way.