For a modern computer, frequent shutdown and startup has a minimal effect on the computer's overall life. The truth is that computers aren't meant to last forever. The average lifespan of a PC is five to seven years, at which point it succumbs to any number of hardware glitches, software viruses, or is rendered obsolete by technological advances. Modern PCs are built to handle 40,000 on-off cycles before failure [source: EnergyStar.gov]. If you shut down your computer at night and start it back up in the morning, you should be good for the next 109 years.
If anything wears down your computer, it's leaving the machine on. Computers build up heat when they are actively processing data and running software. Heat itself can damage internal components, but it also causes your computer's fan to run longer [source: Greenemeier]. The more the fan runs, the quicker it will wear out its bearings [source: Gwinn].