There are several reasons why a computer may run slowly. They include: 1) Many background programs running and taking up memory; 2) A full hard drive with not enough free space; and 3) A fragmented hard drive. Other factors that can slow a computer down are spyware, low memory, an older processor, viruses, and a dirty computer clogged with dust. Taking steps to eliminate unnecessary programs, free up storage, and keep the computer clean can help improve performance.