Make sure you don't have other conflicting styles that are pushing out the width. e.g. If you set the width of the container to 900px and you have several elements inside that container that add up to more than 900px, the container could get pushed out.

Check to see if you have any styles for overflow set to scroll as well.

The easiest way to figure this out could simply be to just remove some elements from the outside in (start with the outer container and work inside) to see which element is causing problems.

If you use percentage based layouts, IE7 has issues with half pixels. As in, it tends to round up some elements. For example, if you have an outer div at 801px and two inner divs at 50%, IE7 will end up wanting the outer div to be at 802px because it ends up rounding the 50% to 401 pixels. (This is based on my observation when I measure it in Photoshop). So this means that you'll have to set the inner ones at 49% each rather than 50%. Otherwise, when you resize the window, the inner divs could end up stacked on each other.

I don't use percentage widths much, so all - please correct me if I'm wrong.

stickygoblin - you can probably find specifics on Microsoft's website somewhere on how IE7 renders HTML. They might have some sort of comparison chart between IE7 and IE8 as well.