As Guest-Tom says, you can most definitely improvise without necessarily knowing what you're doing. Chances are though you'd be using some derivative of some sort of scale in doing so. Nothing, wrong with that. Hell some of what are considered the world's greatest guitarists can't read a note of music, and might even take a few seconds to figure out and tell you what chord(s) they're playing.

Anything you can do to add some sort of spin or variety to the songs you are playing, is improvising. Don't get too caught up in the semantics and the search for the greater understanding of it all. (That'll just get in the way of making music.) Just go for it. There's nothing to lose and everything to gain.