Yeah, I'm not sure about Space Invaders. I don't have either the huey or the CD, so I don't know if either of those have an actual ending. A lot of old games would basically repeat levels, so getting through the first round of levels would count as a clear. But yeah, any of the multimedia stuff doesn't count as a game, or a clear. 
Anything so old that it only has a few screens, I would never count as cleared. Pretty much, my standard is anything that I can honestly say to myself "i beat the game"... Playing through three screens of Popeye doesn't do it for me.
Pretty much if you have doubts, it's probably not a clear.
But there are exceptions to those old games. If you somehow made it to the end of Donkey Kong, or reached the split-screen stage on Pac Man, I think you could count those as cleared. If you'd made it so far into a game that it has wigged out, then I think you've got a pretty good case. lol So, if you managed to play Space Invaders on the PCE until the screen goes wonky, or whatever happens, if anything at all, I would be inclined to count that as a clear. That takes more muscle than clearing a game like, say, Riot Zone or Final Zone II.
Another example would be if you managed to flip the scoreboard. That would also count as a clear in my book.
