Shalmanese is right. In the case of, say, Monkey Island, you’re PROBABLY on safe ground – the game was released by PCGAMER on a CD that came with their magazine a couple years ago. Therefore, you could claim that the game has been released as shareware, and that you personally can’t get hit for anything.
However, copyright is ALWAYS owned by someone, if it exists at all. If a game company goes under, the copyright either reverts to a specific individual (the owner of the company), or to a given creditor who winds up with the thing as a result of liquidation of assets to pay debts.
The only “uncertainty” that can crop up here would be when a creditor does not realize that he owns the copyright… or when a specific “owner” (a stockholder, perhaps) either doesn’t realize he owns the copyright… or doesn’t feel like spending his own money to defend in court his rights to a prehistoric video game that he will never make another cent off of.
“Monkey Island” falls into that category, to some extent. Its copyright is quite clear (LucasArts, as I recall), but considering that the game is ancient, no one really wants to buy it, and Lucasfilms isn’t really into exploiting the characters…
…it’s probably safe enough, even if it WASN’T shareware. The people likely to get busted and sued are the people DISTRIBUTING the thing, not the people downloading it. It’s just not economically feasible at this point to go hunt down every single person who downloads a given item, copyright or no copyright. The best you can hope to do is to sue the DISTRIBUTING sites into oblivion, and scare everyone badly enough that they won’t open distribution sites of their own.
Record companies, take note.
Also note that if the game contained Star Wars characters, LucasArts would be MUCH more likely to sue, if only to make it clear that THEY own Luke Skywalker, and that they will brook no challenge to their ownership of said characters. Disney is famous for suing day care centers for painting Mickey Mouse on their walls, for exactly this reason… not because they’re jerks, but because they HAVE to defend those copyrights… or risk losing them.