Hi Alex,
actually a wild guess but I believe you are working with asp.net webforms, isn't it?
Try specifying your width for the form too:
Code:
form
{
width:100%;
}
Actually, if your html markup hierarchy is as follows
Code:
<html>
<body>
<form>
<div id="PageWrapper">
<div id="MainContent">
.....
the width for html, body, form shouldn't even be specified (although it won't hurt).
Then it's only needed to at it to the styles for #Pagewrapper.