Blind guess (not really all that blind): You're using some sort of application that generates your code, right? Well, first of all stop, HTML isn't all that hard and w3schools can help you to make it even easier.
seconds, why don't you try making the body part fixed?
html:
HTML Code:
<html>
<head>
<title>Page title</title>
<style type="text/css">
div.center
{
width: 800px;
margin-left: auto;
margin-right: auto;
background-color: gray;
}
</style>
</head>
<body>
<div class="center">
something in the center
</div>
</body>
</html>
with something like this you'll have the page in the center and it'll allways have the same width. On all computers.
EDIT: new code, it'll show you the gray background on the center div.