I had this problem the other day when I was building a site for a class project. I found that my problem was that my wrapper div that contained the main content div and the sidebar div wasn’t big enough. I had a wrapper div that was like 910 pixels in width and the main content was 600 and the sidbar was 300 with a 10 pixel left margin. when i resized the browser the sidbar fell below the main content. So to fix it I had to increase the wrapper div by about 20 pixels. That fixed it. Then anything you put after that you have to use the clear:both rule otherwise things could get messed up.
I’ve taken the site down whilst I sort it out.
Better to have no site than a site that doesn’t work in my opinion.
I’ve put a holding page there though using LaunchPad http://www.template-designs.co.uk