Category list to show products when in category

When viewing a sub category or a product within a sub category, is it possible for the category list on the left (by default) to show the other products in the sub category.

That sounds needlessly confusing, let me elaborate!

I have a category called pens. Within pens I have pen 1, pen 2 & pen 3. When the user clicks onto the pens category, I want the list of categories to show the products instead. I also want this to apply if someone clicks on pen 1 so they can jump to pen 2 or 3 from this list, rather than using the prev - next buttons.

I have a secondary navigation list so people can jump between categories.

Am I making any sense? If you can help that would be great!!

Ta,

Scott.