The most common reason a personal computer needs to be reset to factory default settings is the appearance of the infamous "blue screen of death" error message. This has a number of possible causes, but it generally occurs because something has manipulated the operating system's necessary system files.
Malicious viruses can disrupt an operating system, altering system files without the user's consent. Viruses like worms can replicate themselves inside a hard drive until it is completely filled with useless data and unable to operate. Other viruses like Trojan horses can implant themselves in a hard drive, allowing hackers a backdoor into a user's personal files and information. Restoring the PC to factory default settings can help remedy these issues before they cause damage to the computer's hardware or compromise the personal information of the user.
Old computers tend to collect many residual files from installing, running and even uninstalling unnecessary programs. Over time this slows the computer down and leaves less hard drive space for important applications and files. A great way to deal with this is to copy all important files to an external hard drive or large thumb drive and reset to factory default settings. Rebooting the computer, reinstalling necessary programs and reorganizing files creates a better work flow.