I just downloaded Tredsoft's IE program that runs multiple versions of Internet Explorer on your PC. You can choose to install Internet Explorer versions: 3.0, 4.01, 5.01, 5.5, and 6.0 during installation. I just wanted to see what IE 5.5 and 6.0 looked like for the most part, since a lot of people still use IE 6.0 as their main browser, and because I've always run into problems relating to CSS / HTML with sites in Internet Explorer 6.0.
At any rate, the Tredsoft program seems to be a quick and easy way to see what your sites look like in IE 6.0 and below. Go here to get the program: http://tredosoft.com/Multiple_IE
For checking sites out in IE 7.0 and 8.0, I use Internet Explorer 8.0 and then just hit the Compatibility View mode button to see what sites look like in IE 7.0.