Many years ago, a in-law of an in-law asked me to help get them a PC. I bought a box and installed an operating system, gave it to the closer relative to deliver. A couple of days later I got a call from the irate end user Demanding "WHERE's 123!?". The boundary between the operating system and applications was not a concept he understood. You just turned on the box and voila! - spreadsheet!.
Although users as a group are certainly more sophisticated, it is still true today that a lot of the boundaries that developers and techies take for granted are not recognized by users.
My impression is that most users nowadays (or course, that's just the ones I run shoulders with) understand that desktop apps are individual things, because they either pay for them or download them on a case-by-case basis.
However, unless you are targeting an audience with a known level of technical knowledge, I would be very reluctant to make assumptions.