The main problem is that you have set the header to height 100px which is is clearly incorrect as the logo itself is 200px high. Good browsers will assume that anything over the 100px height will be ignored and therefore the nav is placed ignoring the great big logo that is in the way. IE tries to help you out and expands the header to include the logo and thus misplacing the navigation. The header doesn't really need a height anyway as the content will dictate that.
I would set position:relative on the header and remove the height and that will allow you to absolutely position the nav in the top right corner. You can then remove the redundant margin-top from your slider.
background: url(http://actionone.net/wp-content/themes/wp-creativix/images/navbar.png) no-repeat;
[B] /* position: relative;*/[/B]
background: url(http://actionone.net/wp-content/themes/wp-creativix/images/slider-bg.gif) no-repeat;
[B] /*margin-top: 147px;*/[/B]
That seems to get the desired effect in all browsers consistently.