why does my website look all messed up on larger screens?

[monsoonradio]

I have a website at http://www.waxtraxradio.com and on my computer which is a regular screen desktop my website looks fine and centered etc.. but someone sent me a pic of what my website looks on their screen and it is all messed up uncentered etc.. I have the page properties of my website page set to the following.
Center Box Checked
width 960
hight 950
left margin 8
right margin 8
top margin 8
bottom margin 8
the persons screen size where it looks all messed up is
1680 X 1050
I have a regular screen desktop computer.
I recently switched over to sitebuilder from pagebuilder.
Their must be a way to have my page look centered and the same for every computer screen.