IE sees the menu block and the content block as being just a few pixels too wide for the #rap – despite the fact that #rap is 600 and the other two add up to 570 – so it bumps one under the other.
Try reducing the width of #content by 10px ? If that doesn’t work, try increasing the margin of 145 to 155.
Try each on their own if you like, doing a hard refresh (CTRL-F5) when looking after a change.