You have width 100% on content, which makes it stick out 300px to the right of the window, because of the width of #menu_left
. You should make this some pixel value, or change #menu_left
to a % width, say 20%, then content could be 80% and they would fit nicely.
Plus you should remove left: 300px;
from #content
, it will already go where you want it to because of the float: left;
.