Problem with div 100% when horizontal scroll appears

Hey guys,
I have looked for a solution to this, but have not found it.

Basically, i have a header(div) with a fixed height and a 100% width. The div works fine when all of the maincontent fits in the browser window, but when the horizontal scroll bar appears when resizing, the div does not extend the entire width of the page. Help is much appreciated. Thanks!

All block level elements stretch automatically to the 100% width of its container. So you don't need to explicitly specify the width to it.