"well i heard that not turning off your pc will let your pc 'last' more longer, is that true? what kind of pc is suitable for that and what kind is not?"
Generally speaking, turning on and off, on and off, on and off, is bad for any electrical appliance. It burns the wires, circuit and what-not faster (Try it out with a spare lightbulb if you want...)
Also, generally speaking, you'd only want to leave your computer on if you or your computer falls into the following simple category(s):
-A Server computer
-Too slow, and you don't want to waste 5 minutes to wait for it to boot up (Hey, that's what happens with my AMD K-6 266Mhz running Windows 2K... it literally takes 5 minutes or so to start up).
-You believe in the urban myth that turning off your computer DRASTICALLY reduces it's life...
-The computer is a workstation computer that needs to be left on for long periods of time on end to process information (E.g.: A computer used to render 3D Animations with ray-tracing and other CPU intensive tasks)
-Too lazy to turn your computer on and off all the time
Other then that, for an average Joe, there really isn't any reason to leave your computer on.
As for reasons why you might not want to leave it on...:
-Threats of Crackers (CRACKERS, NOT Hackers... Hackers are the "good guys"... Hackers are the people who work in the Pentagon protecting you "innocent" people from vicious CRACKERS), Trojans, Viruses, Worms, and other computer threats and malicious codes.
-You have a fast computer that will boot up in less then 30 seconds, so it doesn't matter if you turn off your computer
-You don't use the computer often, so it's saves power to leave the computer off.
Hmm... seems like my mind is blank. I think the biggest factor to whether or not you want to leave your computer on or off is the follow 3 points:
-Threat of crackers, viruses, trojans, worms, etc... (Basically, your paranoia factor)
-Electric Bills (Don't be surprised to see a drop of at least 30 bucks per month if you leave your computer off for at least 12 hours a day)
-The length of time it takes to boot up your computer (This actually seems to be one of THE main deciding factor for many people I know -- yes, many people I know still have their 486 or 286, including myself).
This wraps up my humble post with a word count of a mere 433 words.
-From your friendly neighbor, Bob Chance.