How to Install Internet Explorer 6 and 7 on the same PC
I want to suggest a very good work from TredoSoft: Multiple IE Installer (aka MultipleIEs) is a free tool that installs all the previous versions of Internet Explorer (from 3.0 to 6.0) on a Windows PC, allowing Internet Explorer 7 users to use older versions of IE on the same computer.
It was very useful for me, because I have to test my sites on Firefox and Internet Explorer 7 but IE 6 is still popular, so I must test everything (at least) on these 3 browsers.