I'm pretty sure the law that says you can keep items sent to you that you didn't order was to protect consumers from being shipped items they didn't want and then being charged unless the item was returned. I've kept several items that came to me "unsolicited" (books and the like), these usually come with a letter saying to look it over and if you like it pay for it, if not send it back. I always simply write on the included bill that I did not order it and am not willing to pay for it, nor am I willing to put in the labor to send it back, and mail it back that way.
I dont think it was intended to cover a situation where you were sent something in error - even IF you are legally not required to send it back, moral forces probably dictate calling them (as you did). At that point the ball is in their court, and I'd just sit on it for a month or so, and if you have heard nothing do with it as you please. If they ever DO come calling asking for it, I'd just say, "oh. yeah, I called and reported it, but was told to keep it. I'ts long gone I gave it to a friend of a friend (or: sold it, or: whatever - preferably the truth) I'd feel fairly certain that they cant force you to pay for something their employee told you to keep with their compliments when you reported it. Frankly it wouldn't be worth their efforts to come after you once the computer is "gone".
of course this is all nothing more than my opinion.