Ok, for what it's worth, uninstalling FSO (just like any other program) is a simple process:
1. Remove program directory.
2. Remove registry entries.
Option two is completely optional, the registry entries really won't do anything if they sit there but if you want to be thorough, use a manual
registry cleaner utility or find the appropriate registry entries yourself through regedit, if you know where they are. But heck, most uninstaller programs don't even clean all the registry entries after them.
Using automagical registry cleaners is a choice that exposes you to a situation where the automagical cleaner software finds registry entries it deems un-necessary, removes them, and programs stop working; I wouldn't ever use them on a machine that I'm working on (referring here to stuff like CCleaner and similar programs).
After the program is removed from its location on hard disk drive and references to it removed from registry, it's no longer on the PC.