3 Column Divs not working

Hi there,
I have the following site:

http://testsite.shops2u.co.uk/categories.php

I've split it effectively into three columns (for the main content) - a left bar, a middle section and a right bar.

Works fine on a large screen, however, when you shrink the window, the text doesn't go with it and I can't find a way around this. (i.e. the right floating bar just falls underneath the middle content).

Any help would be appreciated!

Thanks,

Chris Evans