My buddy, a network administrator, insists on leaving his PC on day and night. His argument is that the system's RAM, CPU, and hard drive are harmed by turning the PC on and off.
I agree that if you're treating the computer like a microwave and flipping the switch a dozen times a day. But turning it on in the morning and off at night? Not a big deal.
Either way, it's still a controversy. But I'll tell you what I do. Turn it off at night. That's because with dual monitor and three hard drives, and even with power saving settings, I'm still burning up energy.
You might want to read Microsoft's "Do you need to turn off your PC at night?" article. The author busts four common myths about turning on and off a computer.
Quick aside: Have you any idea what happens on your desktop in the middle of the night? Check this actual video screen capture for the shocking story.
Free Tool to Save Energy
I'm experimenting with UniBlue's free Local Cooling program. The downloadable software give you an easy way to control the power consumption of a PC. Once loaded, you can set the tool to switch off the monitor, spin down the hard drives, and hibernate to PC after a specific time.

What's cool is seeing the amount of energy you're saving. After a short while, I've saved .139 trees (a Redwood, I hope), 1.265 gallons of gas (probably unleaded), and 2.63 Kwh of power. The site claims over 11,000 users have joined (I'm one of them)
Granted the power saving estimates are just that -- estimates. But it keeps my attention focused on saving energy and possibly helps to decrease global warming.
Do you like the idea of Local Cooling? Tell me in Comments below.
That's a slick idea - instant feedback on your efforts, instead of some nebulous "I'm saving the environment". Now we just need someone to come up with an appliance that you can hang off of your power meter to provide the same feedback, or even a simple online application that allows you to enter in your info from your power bill and give you a month to month environmental impact estimate. Personally I think the appliance would be better as this would be something that could provide instant feedback on your efforts. To be honest, I'm sure I don't do half the things I could to cut back on wasteful energy usage, but how much is simply a result of not thinking about it and how much is true apathy is certainly a valid question. Some sort of instant feedback (or gratification if truth be told) would be a huge step forward in getting people on board.
Personally, I leave my computer on at night - that's when it runs the AVG and Diskkeeper applications and does its auto-backup.
The AVG scan itself takes a little more than two hours to run, so I really don't have time for it to run at any other time.
I also leave mine on for backup and other nightly maintenance. Also, it saves time booting up in the morning!
Steve, you have a GREAT point about saving energy! In the long run I agree with you about turning off equipment and if possible un-plugging them also. Many pieces of equipment can still draw power when turned off and plugged in.
My background is over 19yrs with HP as a CE, break/fix/install work. Customers that turned off there computers had more hrdw failures then customers that left them on due to bearings prob's-heating & cooling.
Turning off your computers at night is a better reason to do backups. I use "Back up NOW! Deluxe" by NewTech Infosystems, Inc (NTI). I think it cost me about $39.
Another safety tip, in ADDITION to doing backups, is to use one disc drive for loading the OS-Windows/MAC/Linux/Unix...(find the fastest/smallest[20-40Gb] internal hard drive for this) and other discs for all apps. If you have an OS failure you only have to reload one disc & do a restore. If an app disc fails you don't lose your OS.
Pat "The Bird Man"
www.birdmanusa.com
Absolutly.I ALWAYS turn off my computer at night.I even turn it off if I am going to be away from it for more than 15 minutes.I have everything electrical,that is computer related in any way,on the same breaker.That way I can kill everything with one switch.The only thing not through the breaker is my cable access.That I physically un plug from the modem.I had my electrician buddy run service from the breaker box to a second breaker in a switch box right next to the switch for the overhead light.That way I can kill them both as I go out the door.My reasoning for the anal retenitive methods?NOBODY can hack into my computer when it is physically disconnected from the outside world.
Let me rephrase something.The only time I use the breaker method is overnight or if I know I am done for the day.
While I agree it's nice to have virus scan/backup/updates/etc. run at night, still something "funny" about leaving machine on all night. So, I use a neat utiltiy called "Intelligent Shutdown" (free or $20 to register so you don't get the annoying screen) to turn off machine. You can schedule it to shut down at a specific time - or when a program ends. Then I use settings in the BIOS to turn on in the morning. This is also very helpful when I'm away from home (i.e. vacation) and might need to access info on computer at home via GoToMyPC or Log Me In - machine turns on and off while I'm not there.