Visualizing problem in ie6

Hello, I am using drupal 6.9 and the latest version of this theme. I've set up the page, and everything seemed to go well, but some users reported they are seeing the left menu in the middle of the page, on top of the main content; it seems it only happens in ie6.
Any ideas about why this could be happening, or how to fix it?
This is my webpage: www.empresarialesuned.com

I attach a browser shot of how it looks with ie6 (wrong) and ie7 (right)

Thank you so much for your help!