Actually, I was thinking if the computer came with a trial AV program on it, start up the trial. Mine came with Norton and it was actually very good, WAY better than Norton used to be. But I see now that nothing came on the machine from reading all the posts.
You might try running Malwarebytes in Safe mode. I have seen some of the techies on this board recommend running it that way. If that still leaves the other user bit behind how about trying a restore back a month or so???
Sorry to butt in here, but are you going online any time you've been doing this and if so do you have a firewall installed and active each time? If not, STOP. No free AV will do a good job of active internet protection. Use another computer and download a free version of zonelabs for starters and install it from a usb thumbdrive.