为什么我的两列在IE7中没有正确对齐？

I'm teaching myself CSS through books and websites. I've made a simple site with a centered box followed by two-columns, and it displays fine in Firefox 3 and Safari 3, but not in IE7. My right column stays on the right, but is pushed down so that it starts where the left column ends. I've seen a lot of blogs about IE hacks, but I don't know which to apply, or where it should be added in my code. Any tips would be greatly appreciated!I'm teaching myself CSS through books and websi