Site appearance on different computer/browsers/resolutions

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I seem to be having some issues with the way my schools website is appearing on our most of our computers. On most of our systems the site will be quite disorganized, text in wrong places, pictures overlaping and it makes me look like I really don't know what I am doing, which I guess is partially true since I am asking this. Any input into how I might go about fixing some of these issues would be greatly appreciated. Thanks!

Http://www.nvcs.org