FortiClient won't install because of old Antivirus remnants.

I spent way too much time trying to solve this, so I hope this helps someone else.

Trying to install FortiClient and it kept failing because it found some residue of Sophos Antivirus.

Every trace of Sophos was thought to be eliminated... until we found this...

Problem You remove antivirus software from an endpoint, but the antivirus continues to show up in antivirus list for that workstation.

Solution The probable cause is that the antivirus did not properly remove itself from the listing in the local workstation's security center. To correct this, run through the following steps on the workstation: