Hi sokogs,
Don't take this the wrong way, but the markup and CSS for that page are so convoluted that I don't even know where to start pinpointing the problem!
For example, you really don't need to use z-index or absolute positioning for such a layout. You also have an awful lot of duplicated and unnecessary rulesets in your CSS which are very hard to follow.
I'd start by stripping the markup back to simple DIVs for the page container, header, menu, sidebars and content. Float the sidebar and content DIVs left. Set the mountain image as a background image on the page container. Use CSS gradients for the page and header backgrounds. Take advantages of CSS cascading and inheritance to avoid duplicating rules across different page elements.
You'll probably find that doing this fixes your IE7/8 issue straight away. If it doesn't, it'll certainly be much easier to isolate the problem.
Hope that helps

Matt
--
Matt Doyle, Elated
Second Edition of my jQuery Mobile book out now! Learn to build mobile web apps. Free sample chapter: http://store.elated.com/