Computers have been built at home for a long time, starting with the pioneer Charles Babbage in the 1820s. A century later, this trend grew when electromechanical relay technology was widely available. The hobby really took off with the early development of microprocessors, and since then many enthusiasts have constructed their own computers.
Early examples include the Altair 8800 from the United States from the late 70's and early 80's. Some were made from kits of components, or simply distributed as board designs. The Altair 8800 pioneered the S-100 bus which somewhat simplified the process. Ultimately, the development of home computers, the IBM PC (and its derivatives and clones), and the industry of specialized component suppliers that grew up around this market in the mid 80's have made building computers much easier. Computer building is no longer limited to specialists.
When PC performance slows to a crawl, a systematic troubleshooting plan will help. Common issues include Processor overheating, Bad RAM, Hard Disk issues, Disk type and interface, BIOS settings, Windows Services, Runaway process, Disk fragmentation, Background applications, etc.