This is because IE 6 has poor css support, IE 7 fair css support, IE 8 better css support, FF 3.5x better yet css support, and so on.
The width should be set in %, not px to fit all kind of screen resolutions.
Not really, because the browser sniffer script and resolution sniffer determine which aspects of which stylesheet to use. And the most common screen resolutions used today are 1024x768 and 1200x800, thus the majority will be able to display 9xx pixels without a horizontal scrollbar.
It's impossible to have sites like msn look the same in all browsers. This is what it looks like in IE6 (1024x768 screen):