You should try to explain what "completely screws up" means, as most of us cannot see it.
In MSIE 7 beta 2, which I use for testing, the sidebar is pressed down and your links in the header ar not visible.
The sidebar problem usually means that there is not enough room for the content and sidebar within the surrounding block. Your main_blck has a width of 718px, and the sum of width for sub_left and sub_right is 718px. You also have padding.
IE treats boxes in a non standard way, which is why the sidebar ( or sub_right ) is pressed down. Try to give the sub_'s a little more room and see if that helps.
Why your header links are invisible, I can't see.
As a further comment: You have a fairly complicated structure in you markaup and accordingly in your CSS.
Good luck!