I wanted to chime in also - I got a chance to work this problem on my computer. I just did a search for "Symantec" on my C: drive. "SEP" would have been next, but I never got that far. I found at least 3 setups for older SEP installations. As I successively eliminated them, I kept clicking the "Refresh" button on the Windows 10 dialog box (the one that told me the 1709 installation couldn't continue without uninstalling SEP). Originally, the dialog showed 4 messages about removing SEP. Then as I removed the setup files, it decreased to 2, and then the dialog box disappeared entirely and the 1709 Fall Creators Edition setup finally continued on and completed successfully. I never had to go through the grueling file comparison that Oren did in the other forum.
BTW, sorry about the double post. When I was refreshing the forum page yesterday, it kept looking like my first post wasn't showing up.