The good news about modern computers is that they are highly modular. Think of them as a series of independent components linked together by an electronic superhighway called a bus. Within various classes of machines, sealed boxes holding hard drives, CD-ROMs, DVD players, and memory can be plugged in or taken out to replace failed units or upgrade them. You can move a functioning hard drive from one machine to another to keep the data, or arrange for a transfer of the data over a network or by direct connection by a cable that links the new and old machine.
If a hard drive fails, though, there is the issue of finding a way to retrieve the information that has been stored on it. You can remove the drive and send it to a company that specializes in emergency recovery of data; sometimes the experts have to physically disassemble the drive in an operating room–like sterile environment, remove the magnetic platters, and install them in special readers. The vast majority of personal computers are based on the PC design pioneered by IBM; a small percentage uses the Apple Macintosh design. The two systems have more in common than they have differences, but you should make certain you understand their peculiarities if you plan to offer repairs on both designs.
You can post your ability in this job by posting flyers and ads in community centers, business supply stores, and schools. Place ads in newspapers and shopping guides. Ask satisfied customers to recommend your services, and offer them a bonus or a discount for business they refer to you.