Getting a certificate error is an issue that can be fixable but if you are unable to install a new browser and you are not even getting the IE’s settings anymore, it means there is a malware/virus entered into your computer. I’m suggesting a few virus removal tools, please apply one-by-one.
Download Adwcleaner. and run its scan once.
Download Zemana and run its scan.
Download HitmanPro and run its scan
Please let me know after that so that you I can suggest further steps.