Testing web sites with different versions of Internet Explorer has always been a  bit painful. Sure you can use different machines, or virtual machines, but that all takes time. However, there’s a tool to take care of testing IE versions 5.5,6,7 and 8: IETester. Its free for private/commercial use, however for commercial use you should buy a license.

On a related note:

• Check out this article by Lins about why we should stop developing for IE6
• Check out this article by NetTuts+ about 9 common IE bugs and how to fix them