The hard drive is the single most important device behind the mass acceptance of personal computers in the home and workplace. The ability to save significant amounts of data within a computer itself, rather than being forced to place it on external media finally fulfilled the potential that the PC had never quite lived up to, at least as a business tool. Hard disk storage ability has increased massively since the early days of the technology, and will likely continue to increase in the future as drive's cost per megabyte of storage drops equally fast.
Unfortunately, the underlying technology of hard drives has changed little in the years since their invention, meaning that their essential weakness still exists. The simple fact is that hard disks are mechanical devices with moving parts, and as such, will fail eventually and inevitably.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips