Looking for help with page alignment issue

I'm a beginner in dealing with Drupal and themes, so any assistance someone could provide would be great (and I'm sorry if this is a dumb question, I couldn't find an answer elsewhere, and I'm also sorry if this has already been answered). I am using a modified version of the SpreadFirefox theme for my site, www.nonprofitcongress.org. I'm running into an issue where (and this seems to affect Internet Explorer more than Firefox for some reason) sometimes the middle column on my site get pushed down to the bottom of the page (giving the illusion that the page is blank unless someone scrolls down). It seems that this happens if there is something in the center column that is wide, which affects my event listing page and a few of my forms, even with a 1024x768 resolution, but seems to affect a lot more in an 800x600 resolution. Does anyone know what line of code in the layout.css I need to be tinkering with to make sure that everything doesn't get pushed to the bottom of the page? Thanks in advance to anyone who can provide any assistance in this matter.