With that question, you have strayed into a long-running squabble among computer users. One side argues that you should turn the computer off when you are not using it. Even when you are not at the PC, the hard drive's spindle keeps spinning, adding unnecessarily to the wear and tear on the drive. The computer can overheat, this argument continues, particularly in winter, when the furnace runs.
Heat is a leading PC killer.
Bah, says the other side. Turning the computer on and off unnecessarily stresses the PC's mechanical parts, especially the hard drive, shortening their lives. Electronic parts are at risk, too, because extra jolts of electricity course through them every time the computer is booted. If you are on the PC several times a day for an hour or so at various times, but do not use it continuously, keep it on. If you use the computer hard all day long, you should reboot at least once a day.