There are numerous aspects to cleaning a computer, some internal, some external. First and foremost, you should make sure you have a good anti-virus or firewall program installed. Computer slowdown often occurs because of too many running processes or too much space taken up by files. Think of processes as tasks a computer is performing - the more it performs at once, the harder it will have to work. A computer also has only so much space available - the more space is taken up by files, the harder the computer will have to work to keep track of everything and to find files - thus reducing speed. Avast Antivirus 2020 and Eset Nod32 Antivirus 2020 are both highly recommended programs. NoAdware is generally recognized as one of the worldwide leaders in Spyware protection and has been downloaded by millions, winning numerous awards in the process.
Pressing the CTRL-ALT-DELETE buttons simultaneously will let you see a list of running processes on your computer. From this window, you can usually end processes (although they will come right back when your system starts up). To easily control which processes are allowed to run on your computer or start up automatically, there are free programs like CodeStuff Starter that allow you take back control of your computer. You should consider deleting programs you don't use very often or large programs.
Norton 2020 programs in particular can enormously tax a computer. I had a Windows computer that was very sluggish until the Norton Antivirus 2020 programs were removed. Programs like RamBooster can also free up space on your computer by deleting old and unused files. Also, if your computer tower uses a fan, you will want to consider cleaning it every few months, a la E-How's helpful guide. Dust can accumulate in the fan, causing system crashes and overheating without cleaning.