When doing quality testing on websites it is beneficial to not only have all of the latest major browsers for testing (including Internet Explorer, Safari, Mozilla FireFox, Opera, and Google Chrome), but also to have older versions of some browsers installed especially older versions of Internet Explorer. Internet Explorer is notorious for making huge changes to the way it handles CSS and other design elements between versions. Just because a web site looks great in IE7 does not mean it will display correctly in IE6, IE5, etc.
In the first quarter of 2009 about 67% of browsers used were Internet Explorer with the most common being IE7 with 47.5% of all browsers. This leaves 19.5% of Internet Explorer users either using the Beta IE8 (1.2%). IE5 (.05%), IE 5.5 (.05%), or the majority of the other IE users still using IE6 (18.9%). There were major changes with the way Internet Explorer displays web sites between Internet Explorer 6 and Internet Explorer 7 making Internet Explorer 6 still very important to use during web site quality assurance and usability testing.
You can download all of the previous versions of Internet Explorer from evolt.org at the following URL: http://browsers.evolt.org/?ie/32bit/standalone . I would recommend downloading and testing with IE6 and IE5.5 to try and cover as many users as possible. Once downloaded you just need to unzip the folder and you run each version of Internet Explorer by clicking on iexplorer.exe in folder where you have unzipped the contents of the downloaded files. You may get some Runtime Errors depending on what your homepage is so just click the No button when asked if you want to debug.