CSS will be the death of me

You know, I really like CSS, and it makes live easier in so many ways. Unfortunately, because I haven't spent a ton of time with it, it's also a pain.

Formatting is easy, and works predictably across all browsers. It's the layout stuff that often frustrates me. This time around, I'm trying to plug in POP Forums into an existing page. I thought the CSS I ship with the UI was pretty easy going, and I've dropped it in other layouts before, but it's not cooperating with my current project. The biggest concern is that setting the forum tables and/or divs to 100% makes them 100% starting from the left margin, which means they bleed off the right side.

I know it's something simple, but like regular expressions, I frequently can't concentrate long enough to get the right formula!