Good tips above, ESPECIALLY to have a backup just in case it all goes horribly wrong.
I have been installing and uninstalling various security products for a few weeks now trying to find a product lineup to stick with so I try and keep things clean. I finally just said screw it and did a clean windows 10 install this week anyway. It wasn't necessary, but the speedy feel of a clean install is great.
For removing something I really want to get rid of like an old AV:
1) Do normal uninstall with either Geek Uninstaller Free/Revo Uninstaller Free (my preference is Geek) If using Geek, use ONLY the free version, the paid one seems to be a different program entirely. Geek Free:
Download | GeekUninstaller Revo Free:
Download Revo Uninstaller Freeware - Free and Full Download - Uninstall software, remove programs, solve uninstall problems
2) Run any av removal tools offered by the product you are uninstalling OR even available at their competitors -> I know Eset maintains a great list of av uninstaller tools presumably for folks to move to Eset AV. It's a great compiled list of AV uninstallers in one easy to reference place:
Uninstallers (removal tools) for common Windows antivirus software—ESET Knowledgebase
3) Run CCleaner (pick the Registry tab to scan, by default it cleans files) and/or privaZer (with the most conservative (choose basic user) default settings for safety) to rid the registry of anything else and any abandoned leftover files etc. Ccleaner Free
CCleaner - Standard2. privaZer
Free PC cleaner. Be VERY careful with privaZer as it's much more powerful and aggressive than CCleaner. I use it at the lowest possible settings and even then I make sure I do a full restore point before even launching it.
Something almost always gets left behind, but if you follow these steps, restarting as directed by each tool, I think what is left over will have negligible impact on future installs of any av product.