lorax - 12:40 pm on Aug 31, 2012 (gmt 0)

Welcome to WebmasterWorld mamamiaw,
You could do this using a combination of a categories plugin and the widget logic plugin. You'd set up the categories to display based on the main cat that was open then use Widget logic [wordpress.org] to show the plugin or not based on the category that was currently being viewed.

The logic would use the is_category [codex.wordpress.org] and in_category [codex.wordpress.org] functions.