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rabideau: while some great advice, your later statement is not exactly true.
Just setting content to 100% is often not sufficient, as many themes use margins to place the content in the center.
MarcDeMesel you will need to check your
theme.html and
style.css files of your theme. Depending on theme, it
could involve quite some fidling around.
And also search the forums, there has been a similar question just a few days ago, which DonXoops gave a more detailed explanation, than my lazzy attempt