(Windows)
Slow computers are often caused by too many programs or processes running. These programs can be spyware or bloatware.
Bloatware can be removed using the uninstall feature in Windows Control Panel.
Spyware once in a while can be removed in the uninstall, but many times needs to be removed with an antispyware program.
Sometimes programs need to be removed in safe mode.
Running disk defragmenter can also speed things up sometimes, but these days this usually isn’t the problem.
An antivirus may slow down a computer, however it is a must for Windows. Choose an antivirus that does not have high system requirements and make sure your computer meets those requirements. Only have one antivirus on your computer. You may have to tweak the preferences on the antivirus.
Many times the computer is so slow it is best to use the recovery discs/partition/os disc to wipe everything out and start fresh – make sure you back up your important data files first if possible.
Many times on the hardware side, more memory can help speed things up (using any os). There can be other hardware slowing things down, but this is the most common.