From my original post:
NB: During the installation process you'll be offered a free trial of the Premium version. The box will be pre-ticked. Click to remove that tick!
To fix the problem now, download and run this file:
http://downloads.malwarebytes.org/file/mbam_clean
(It's the official tool for completely uninstalling Malwarebytes Anti-Malware).
Then RESTART YOUR COMPUTER. (That's IMPORTANT!)
Then download and install the FREE version of Malwarebytes Anti-Malware again, this time taking great care to remove the tick that I told you about!!!
https://www.malwarebytes.org/antimalware/
NB: There's an important difference between the Premium version (which you've been using as a free trial) and the free version. The Premium version 'runs in the background', so that it's always looking for anything nasty which is trying to find its way onto your computer. The free version is only 'run on demand', so it will do absolutely NOTHING unless you remember to run it occasionally. (I suggest once per fortnight or, of course, immediately if you suspect that there's some malware on your computer).