Is there a way to have multiple versions of Internet Explorer installed?

Try using Multiple IE. It won't work in Vista unfortunately though.

Also, you may find this site useful, which allows you to test using all versions of IE over the net (using screenshots). I've found it handy and accurate. http://ipinfo.info/netrenderer/index.php

What version of Windows are you using? If you're using XP, you can download a standalone version of IE7 here. I think the blog also has versions of IE6 as well.

Basically googling IE6 IE7 standalone will help too.

If you’re on a Mac, you can run different versions of Internet Explorer in separate Parallels virtual machines.