PC hardware experts generally agree that the act of turning on a personal computer, with the subsequent surge of electricity through the electronic components, causes the most long-term "wear and tear" and is the primary contributor to hardware failures.
In an ideal situation, the PC would be left on 24 hours a day, 365 days a year. On the other hand, a typical PC with CPU, monitor and printer generates about $100 a year in electricity costs if left on 24 hours a day. Also, rebooting (powering off and then on) a PC allows the operating system to "refresh" generally resulting in fewer "lockups" caused by memory-utilization issues.
Balancing these factors, Information Technology Services recommends turning off employee PCs at the end of the workday and turning off computer lab PCs when the room is closed and locked at the end of the instructional day. We believe this recommendation strikes an effective balance between the electrical cost, the operating-system memory refresh and long-term equipment-failure rates.
However, though we recommend leaving the PC turned on throughout the workday, it is still very important from a security perspective to "lock" the PC (in Windows, press CTRL-ALT-DELETE or click the "lock workstation" icon in the system tray along the bottom of the screen) when left unattended for more than a few minutes.