You are mistaken. Once a program is uninstalled, it is gone from your system. Odd little bits may remain, but it's along the lines of the adhesive left behind by a sticker. The analogy doesn't quite work as well with computers, but hopefully you get the idea.

So unless you have something a bit more specific you're worried about, you have gotten your head full of some bad info. Wherever it is you came by this little nugget, don't go there/listen to that person again.