Turning it off or keeping it on
One very common question that I get as a computer technician is “Should I turn off my computer or 
just leave it on all the time?” Since this isn’t a black/white question let me take a moment to explain…
This is no real simple answer to the question, but I can offer some advice that might help. Believe it or not computer monitors use more power than you think. In my research I found that CRT [bigger tv style] monitors use close to twice the amount of electricity as a normal light bulb. If you have a LCD [thin type] monitor it consumes about one forth the amount of electricity as the bigger CRT monitors. One piece of advice that I can offer is to simply turn off your monitor when you are not using your computer. It doesn’t harm the monitor or the computer to do this, so why not? The computer tower itself is a different story.
The biggest argument when it comes to PCs is that turning them on and off all the time stresses the components inside of them. I can’t agree or disagree with it because I haven’t done any testing myself, but its about a 50/50 argument as far as my research goes. Whichever side you take, keep in mind that your computer doesn’t have to be completely powered off to save money. You can easily adjust the power settings in the control panel of a Windows based PC so it will automatically turn off your monitor and hard drives. If this doesn’t sound like something you want to do then remember that you can always hibernate [saves what you are working on to the hard drive and powers off the PC; boots quicker than normal] or put your computer into standby [consumes power as the PC is kept in a low power state; computer turns back on almost instantly].
Since this is all about saving electricity, why not turn off your PC? If you are only on your computer after work, or in the morning to check your email, go ahead, turn it off. If you use your computer several times throughout the day and don’t like to wait while it starts up because its ‘too slow’, then consider using standby or hibernation, as they are a great way to save electricity, but also have your computer turn on faster than normal.
I personally combine all of these pointers to save electricity. I have one server that is kept on all the time, but the monitor connected to it is always turned off. I have one laptop that I shutdown when I am not using it, and I have another that I keep on standby so I can be working instantly. My server is a 10+ year old abused laptop and it runs fine. I have a desktop PC that is on all the time and has been for years; only shutting it down when there is a bad storm and it hasn’t had any more issues than any of my other machines.
Everyone will tell you something else when it comes to keeping your computer on or turning it off and all you can do is listen to what they have to say. If your PCs main reason for staying on all the time is a slow startup, remember you have power saving options available, and not necessarily at the sacrifice of speed.