as far as playing on the computer goes, we have a game for our son (i think it's fisher price) that the "keyboard" for the kid fits over the normal keyboard. you just run the program and it's all types of games and learning for them. he loves it. we were looking into the V-Tech someone else mentioned too just because he seems to be getting too old for this one. but 18 months should be the perfect age for the fisher price one. and it's reasonably priced.
As far as the poster that mentioned that they need to respect your things, they're just trying to imitate what you are doing in my opinion. if they see you on it, they'll want to do it too. if they want to play on the computer, get them a game and let them play! i can't believe how much my son learned on the computer and he's only 2 1/2!