I’m sorry but I do not have the time to check the complete CSS, but my guess is that there is a block element (h1, h2, hX, div, img, ul, ...) in your right column, that is too wide. IE would do the thing you’re describing.
Other possibility is that there are two block elements, one floating left, other floating right and both are 50% wide. IE has this strange behaviour, where you should set one of the block 49.9% wide, the other can remain 50%, and the problems are solved.