Website looks different on different computers (even on ipod touch)

I created a website using iweb and I understand that different browsers show things differently but on my computer (MAC) when I open it in Firefox and Safari I see the same thing, but when friends who are also using Firefox, Safari, and Macs check it out they see something different. I checked on my work computer which is a PC and used Firefox and i still saw the same thing (the way it should look). A friend sent me a screen shot of what she was seeing which was basically what shoudl have been the entire webpage but it showed in the upper left corner of the browser instead of it filling up the whole page. I then went to the website on my ipod touch and saw what she was seeing. I had a few other friends check it out, mostly macs but i had them look with both Safari and Firefox and some people saw it the correct way and some didnt. Any ideas?