Hi, i'm looking to do a webpage with a header (something like 100px height) and a background full width/height on the rest of the page, but not under the header, how to do that ?

wha?
got an example?

Have the header in a div and set it's z-index attribute higher than the rest of the page and give it its own background maybe?

I'm sure there are multiple ways to do this, depending on how you've implemented the header/background.

Mostly with CSS.

Here's a quick example: http://jsfiddle.net/B7Ksn/

You could even set the background of the body to an image and not have to worry about the header background.