I couldn't find any differences between your CSS file and hec.css that would cause this, but came upon an image tag in your mod-OrderCDs module that might: img src="images/none.gif" width="100%" height="0". IE6 could be interpreting that width specification as "make this image (and thus the column) as wide as possible." In any case, this image tag isn't needed, as far as I can tell (unless used for counting purposes?), and might as well be deleted.
Update: I just noticed that "images/none.gif" is part of the default Tiki page, apparently used by the column show/hide script. (My own themes haven't used this so I yad forgotten about it.) So I guess that probably isn't the cause of the problem.
I would try turning off column-visibility switching, though, to see if IE is having trouble with that.