The problem comes from the fact that different browsers parse HTML and CSS differently. You can search on the internet for some tips on how to make your site look relatively the same on different browsers.
One way is using percentages instead of fixed widths to make the page automatically expand to the size of the users screen.
You can use the following free tool to assist you in getting your website correctly displayed across multiple browsers and operating systems:
If you want to see how your page displays across multiple browsers and operating systems without installing them, you will find the following website very useful: http://browsershots.org/