Agree with both answers above. An ever increasing problem is, installed freeware comes with attachments, and it's not that easy to notice what the whole package is. And, it's not only freeware related, paid for software also carry extras with them. Installing becomes more and more responsible.
I would suggest one of the basic methods: go to Control Panel\All Control Panel Items\Recovery > Open System Restore, and pick a point / date you think is before problems appeared. In System Restore you may not see more than one, but you have the option to "Show more restore points", as well as "Scan for affected programs".
Finally, if you don't find anything with antivirus / malware scanners, it's probably because you have installed something in a legitimate way - what you don't want, is there because somehow, at some point, you wanted it. Look for the needle in the haystack!
Best wishes.