You have set the fieldset to clear both so it does what you asked it and clears your floated left column.
In a fixed width two column layout you should simply float both columns (instead of using the margin-left technique) with appropriate widths and avoid this issue as clearing of elements inside a float is confined to that parent only and has no effect outside.
Either float both columns or instead nest an inner element inside the content section that is floated and will contain all that right side content. (Or use overflow hidden on Content and apply haslayout.)
The two column float is the most robust solution:)