Quite rightly, this has been posted everywhere, so consider this an aide-memoir.
As Web designers, we all know that Microsoft doesn’t let you install different versions of Internet Explorer side-by-side. This makes testing across versions a nightmare, usually resulting in maintaining dedicated testing machines with out-of-date versions of Windows in order to test older versions of Internet Explorer.
Well that’s all changed. Joe Maddalone of Insert Title Web Designs noticed that Microsoft used a neat DLL substitution trick in order to make available test versions of Internet Explorer modified to comply with the Eolas ‘906 patent that could run side-by-side with the unmodified version of the browser. Joe found that the same trick can be used to run different versions of IE back to version 3, and he has published an excellent guide on how to do so.
Ryan Parman has taken this a step further and provided convenient, pre-packaged downloads of Internet Explorer 3, 4.01, 5.01 SP2, and 5.5 SP2 with Joe’s steps already applied. Be quick though, because Microsoft probably won’t appreciate IE being made available packaged like this.
As reported by Sitepoint, the DLL technique isn’t entirely without side-effects but minor functionality problems manifested in older browsers are a worthwhile sacrifice for the benefits of reducing the size of your test lab.