Page and background not centering

I'm a beginner CSS coder, so I'm sure I've done something extremely stupid:

I have a wrapper div that is 850px wide. I have set margins to: 0 auto;

It looks fine on a Mac running Firefox or Explorer, but when viewed on IE 5 or 6 on a PC, the background image is off and the site is flush left - not centered.

My header div is 800px wide and centered, and my main content dive is 800px and centered, so the background image of the wrapper will show on sides and top evenly.

Here's the url: http://www.hudsontrailers.com

Here's the css:

body {
background-color:#003300;
font-family:Palantino "Times New Roman", Times, serif;
font-size: 1em;
text-align:left;
}

#wrapper {
background-image:url(../images/bkgnd.jpg);
background-attachment:fixed;
width: 850px;
height;
margin: 25px auto;
padding:25px 0;
}

#header {
background-color:#003300;
border-top:medium solid #FFFFFF;
border-right: medium solid #FFFFFF;
border-left: medium solid #FFFFFF;
width: 800px;
margin: 0 auto;
padding:0;

}
#main {
clear:both;
background-image:url(../images/images/main_bkgnd.jpg);
width:800px;
margin:0 auto;
padding:0;
border-left: medium solid #FFFFFF;
border-right: medium solid #FFFFFF;
border-bottom: medium solid #FFFFFF;
}

Any help would be greatly appreciated.