Hi Dominique,
The site is showing fine for me IE & IE9 but not in IE9 Compatibility Mode (which is almost equivalent to IE7).
I noticed that the "StyleHeader.css" and "StyleFooter.css" files are being referenced from within your layout.master. This isn't the right way to reference style sheets in mojoPortal. These files should be referenced from within the style.config file of the skin. This may or may not be part of the problem.
To fix IE specific issues, you should add css rules to the StyleIE7.css file in the skin directory. As far as what to add to fix the issues you are having, it will take some work. IE7 is stubborn sometimes. You can start off with the following.
To get the logo placed correctly:
DIV.logo { left: 0; top: 0; }
To fix the menu:
.art-nav { top: 0; }
There are some z-index issues with the menu but if you add the above I'll take another look to see if I can fix it.
HTH,
Joe D.