Many programs, when un-installed, leave some 'breadcrumbs' in the registry so that a 'trial version' cannot be endlessly installed, used till run out of trial period, un-installed, re-installed, etc.

I don't know how you clean this space wasting rubbish out. If a trial runs out on me and I haven't bought it, I rarely would bother with the above conniptions anyway. Most trials don't even last one day with me - so now I don't even bother trialing many things at all.