Float problems with IE - postleft & postright

Hi,

And first, thx for this very cool board !
I have managed to integrate the board in my website (technical support users).
http://faq.easyclic.fr

All is okay with this build, even the color and position with css. I have only one problem :

In viewtopic.php, posts are separated in two columns (postleft and postright both in a box and inbox class).
Normally, the float:left is used for postleft and postright take the rest of the place.

Integrated in my website, all is okay under Firefox and Mozilla, but with IE, there is a problem, the postright don't take place at right of the postleft but under it. You can see the bug here : http://faq.easyclic.fr/viewtopic.php?pid=3

I have try more solutions, even a clear:both or put a negative margin to post right, an positive to postleft, this second solutions works to move this two elements but not as perfect as the initial PunBB.

Thanks you very much for you help.