Not having seen the skin - I wonder if it's the html design used that's done that - have you tried to take the search out of the top menu to see if that' doesn't help close the gap. The gap looks similar to the width of the search token, meaning you have to reconfigure your skin layout if that is the case. It's not a dnn issue, but how the html has been done if that is the case.

Nina Meiers
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I don't know what the proper solution is, and I'm hesistant to suggest a stupid IE-only fix, but I was forced to do this as well.

In my portal.css, I put the following:

#dnn_LeftPane {
width:800px;
}

Setting your main pane to something huge will squish the surrounding modules to the side. If 800px doesn't work, try something higher. In my testing, this method has never forced horizontal scrollbars on the user.

Sorry I couldn't give you a non-hacky answer.