Well, <!--[if lt IE 6]> is going to target IE lower than 6 - try either <!--[if lte
IE 6]> (lower than or equal to IE6) or <!--[if lt IE 7]> (less than IE7). (The table is 95% width in IE5.5)
That said, the right column drops in virtually everything at small resolutions, in FF about 863px wide. This is because although you have the column widths set as percentages, at some point the images in the columns become wider than the percentage. In IE6 this forces the column to extend, hence the float drop occurs earlier.
You could just set the width of your table to be fixed at say 1000px, which takes care of all of this.
Incidentally, have a look here
for a good three column layout that avoids tables.