Forum Width problem

Hi.
I´m having some problem with the forum width.

When i´m in the forum index, the witdth is right, 100%. But when I´m in the forum categorie, the width is shorter.

I have some screen shots for making it clearer:

Forum index:

http://img14.imageshack.us/img14/7036/17168039.jpg

Categorie:

http://img14.imageshack.us/img14/601/20165780.jpg

I´d appreciate some help...!! Thanks in advance...!