Is it good to leave your computer on all the time or turn it off all the time? The reason I'm asking because I heard somewhere that the power supply for the computer lasts longer.

I have been told that shutting the computer off when not in use is a good thing. It was something related to items on the screen being burned into the screen. I do not know that for absolute positive fact. But the computer tech at my last job told everyone that when he installed the new systems. I am sure there will be other opinions, and others more insightful.

It won't do your computer any harm to leave it on for a long time, unless it's a Laptop - laptops get hot and shouldn't be left on indefininitely.

Mine is often switched on for 12 hours a day, and the computers at work are on 24/7.
Your monitor (screen) should be switched off when not in use - although they're a lot less prone to the an image burning into the screen these days (think of how a poster in a window will fade in bright sunlight - it's much the same principle) they will last longer; and it saves electricity of course.

I have been pretty much leaving mine on 24/7 for years. I would recommend letting the monitor go into power saver. You can do the same for the hard drive, but I don't usually go that route. With most people have cable or DSL these days it is pretty common to keep the thing running just to stay online.

Our Power Mac must be at least 10 years old. It only gets shut off for thunder storms. I have always left my computers on going back to my 16 K RS Color Computer. Every one was still running well into its obsolescence. Our son was embarrassed in the mid 90's at Carnegie Melon to be getting email from home sent from a, then upgraded to 64 K computer.

I still love the old Mac for the ease with it does simple little tasks. Sledge hammers don't make good tack hammers.

This issue has been a subject of debate for about 20 years. The facts of the matter are as follows:

There is more wear and tear on the system during the power up and power down processes then while the machine is running.

Ergo, it doesn't make sense to frequently turn it on and off.

Older monitors did have a tendency to suffer burn in, but with modern LCD monitors and screensavers, that's no longer an issue.

My main PC stays on 24/7, but my laptops get turned off when not going to be used for several hours.

I leave mine on 24/7 also, I always heard when you power it on it uses a lot more power than just leaving it run, also as mentionned above power on and off cycles are said to put more wear on the computer parts.

The only time I ever turn my desktop computer off is if there is a storm coming. I am always remotely getting into my computer from work though, so there is actually a reason for me ;)