If you've already got the free version of Malwarebytes Anti-Malware on your computer, simply run that.
However a fresh download of the program now comes with a free trial of the 'Pro' version, which seems to slow some computers down, so I don't recommend downloading it.
Instead, I suggest using AdwCleaner (which is 'out of the same stable' anyway). There's no installation process involved; you simply download the file and run it directly:
https://www.malwarebytes.com/adwcleaner/