whats happening is your not thinking about the box model properly. Your wrapper is set to 940 px, and your 3 columns are "nav-column 135px wide, "main-column" 580px wide, and "sidebar" 170px wide totalling 885px. The problem is that you’ve added padding to the first two. Padding increases the size outward from the width applied. so your "nav-column" is 135+5px, "main-column" 580+90px, and "sidebar" 170px, totalling 980px 40px over your allowed width (set by the wrapper"
So to fix, you could set the width to less say 130 px? and float all three columns left. (instead of the last one right.)