Several years ago I had a PC game ... I don't recall where I got it or what it was called.

You started with a small box with two balls bouncing around and when you clicked in the box, walls grew to block off the bouncing balls. When you cleared two balls, the next box had three, and so on. The objective was to isolate each ball into its own box. Does this ring a bell to anyone?

Thanks

Sounds like Qix (or a clone):

http://en.wikipedia.org/wiki/Qix

Barrack for the Mac sounds closer to your description:

http://en.wikipedia.org/wiki/Barrack_%28video_game%29

I bet this is the one you've played (JezzBall):

http://en.wikipedia.org/wiki/Jezzball

Yes, yes! Jezzball is the one ... thanks!