without seeing an actual example of your problem youre only going to get a generic answer, and its the same exact answer that 1000 other "sidebar fallen" posts have gotten.
1. Your attempting to insert something into a fixed width div that exceeds that fixed width. In other words, you cannot put a "thing" that is 600px wide into a fixed width div thats only 400px wide. It doesnt matter if the theing is an image, a .swf, a.fla, .. if its too big, its too big.
You also need to bear in mind, that IE (6) handles padding and margins differently than all those other browsers you mentioned. THAT must be taken into account when determining how much room you have available within any fixed width div.
2. youre pasting from word.
3. Youre (mis)using italics, and havent taken into account the fact that IE6 handles italics slightly different than regular text.
and so on, and so on, and so on.
lastly, and again, without actually seeing, this is generic advice.. if your site doesnt validate.. well then, it's pretty much a crap shoot how any browser will display it.