Swaps?

I'm really loving this game, but I don't know how to earn letter swaps. Sometimes a number shows in the swaps area at the lower right, but usually my number of swaps is 0. I understand how to earn shuffles but not swaps. Can anyone help me?

Also, I'm currently playing on XP with no problems, but I'll be switching to a new computer with Windows 7. It looks like it cannot be played on Windows 7, judging by the comments in the technical problems section of this forum. Has there been a fix for this yet? This is a great game, and I hope to continue to be able to play it.

Swaps seem to disappear when you turn the game off after a session of playing. I think you get 5 swaps per world.