Display problems with Internet Explorer

[Isura]

I worked on a css for my site and was checked it with firefox and internet explorer. In firefox everything worked fine. Only the internet explorer have some display problems with a few sites. First I thought I have made some errors in my css and changed it back to one of the default ones. That doesn't fix the problem. This is how the page should look like:


The content is centered in the browser window.

When I visit messages.php, myadopts.php or profile.php everything move to the left site:

And the drop down menu doesn't work anymore.
When I look at the profile from a user the tabs ('about me', 'friends',...) are not clickable and the menubar disappears (also happened in firefox).

I also tried it with the other default templates --> same problem.

Have anyone an idea how to fix this?