Basically, it depends on the computer. Some reasons to NOT leave it on all the time are if the situation is:

If you're concerned about electricity usage.

If the computer gets hot. This can be bad because if you run a load of programs and you can hear your fans roaring, it's a good idea to shut it down or at least put in on standby or hibernate once and a while so it can cool down. This is especially important if you're in a warm environment. Laptops are more likely to get hot quickly. For a desktop that get's hot because of a lot of processes, overclocking, or warm air in the room, it's sometimes a good idea to take the casing off so that it can breathe (unless there's a whole lot of dust in the room).

If the room/building is dirty/dusty. Dust and grime will clog the screen for your fan, and if this happens, you're gonna suffer. Usually, air conditioned rooms have cleaner air, but in that case it's also not too hot. If it's not air conditioned, it's more likely to get hot and there's probably more dust. It seems to be a no win situation, but this can depend on other things too. Just make sure your screen and fans are clean and clear so that you don't overheat.

<--- Someone please undo that negative point for me , thanks.

The only good reasons to leave the computer on for 24/7 is if you know that you are running on a stable Operating System such as Linux or any UNIX based OS including MINIX, FreeBSD. If you're running a server using your computer. Don't want to kill your system as fast. Your computer heats up quickly (like my Samsung NV5000 laptop).

Reasons not to leave on is:

If your OS is unstable as Windows. Easily infiltrated because you don't have a firewall or anitvirus. If your parents don't know that shutting off and turning on a computer takes as much energy to operate. If you're on vacation (except your laptop since you bring it with you). Your batteries are low. Electric company shuts off your power because you spent your money drinking beer or smoking instead and the lamest reason, you hate the fan's whizzing noises.

xboxrulz

Hi guys,
In general if your going to use the computer periodically during the day , then leave it on. Turn it off when you'r done for the day & give it a rest. This generally optimizes the trade offs between mechanical part wear out (eg. hard drive bearings) on repeated startups and "burnout" of electronic circuitry due to temp buildup. Also, "optimizes" the extra power usage from too frequent reboots.
Other considerations mostly all apply to thermal buildup which can best be handled thru improved ventilation, AC, & upgraded cooling system add ons (eg. fans, case mods, H2o coolers etc.)
Just my 2 cents
RGPHNX

I never leave my computer on longer then i have to. I don't do it for the following reasons:

1. It absolutely kills your electricity bill - A PC uses a fair bit of electricity
2. It increases the chances of catching anything - you can't catch anything if you are not connected to the internet
3. I am paranoid about leaving anything turned on if i can't keep a check on it - Just the type of person i am

I can't understand personally what is so difficult about turning the PC off, and then turning it back on when you want to use it next.

The only time i would leave my PC on overnight is if i was to download a large file, but i hardly ever do, so that should never be a problem for me.

I was running an older computer 24/7 with a shoutcast server on it using windows ME....It ran pretty good and all but I wish I used fedora instead, I was too lazy to go and set everything up on linux which takes a while to install things and get everything working....I think that as long as your computer has space to "breathe" and isnt in a highly dusty area, you should be fine with it running, but make sure you check on it every once in a while to make sure it is still working and not locked up.... Right now my computer is off due to it have been locked up for a while and my server not working due to that. Since spring break is so close, I plan on installing fedora or a different linux OS to run smoothly and have more control over it..The only thing I have to figure out is how to change my IP to a static IP in linux and how to get everything running correctly.... My only suggestion is not to use the computer you need to work with because if you run a server and try to do other things on it, you might result in a lock down or super slowness...

I keep my PC turned all day but I turn it off at night so that viruses / worms don't get in. Also, leaving it on 24/7 (onmine anyway) makes it run slower in the morning.

I think if you don't need your computer and your going somewhere for a while just turn it off so it could rest. Especially if the fan is loud when your like sleeping lol. Sometimes they overheat but that's rare.

My computer is quite old now and is running on Windows ME. Now that I am on holidays I will wake up in the morning around 9:00 am and leave my PC on untill I go to bed which is normally around 2:00 am the next day. If I leave my PC on over night then when I wake up in the morning it will be going slower than your gran running in a race.

whoops, I spotted a mistake on my original post about the heating up the NV5000 laptop, it should be reasons to not to keep it on the whole day.

xboxrulz