20" intel iMac. Mighty Mouse

I want to disable the dashboard feature. I don't want to use widgets but mighty-mouse somehow brings up the dashboard and dulls down the screen and I cannot work out what I am doing with the mouse that opens the dashboard. Any ideas for me on this one?

I like the mouse's ability to scroll sideways, so I would rather keep it than buying a $7.00 Harvey Norman two button optical mouse.

Kev