Having trouble fixing my column widths with a flash banner

https://nbnmagazine.com/wordpress-mu/

Looks great fully expanded, but when I minimize the browser window the flash banner floats over the second column totally messing up the aesthetic.

I’ve been trying to fix the column width on the first columnby editing a bunch of the css files, and hard coding the xhtml, but no luck.

Any suggestions? Do you know exactly what files I should be editing?