Only one version of Internet Explorer (IE) can exist on a single windows installation by default. I had previously used Multiple IE as a way of testing web pages on older versions of IE. This allows you to have IE3, IE4, IE5, IE5.5, and IE6 installed alongside your existing IE7 or IE8 install. You can even run them concurrently.
I don’t test pages on anything earlier than IE6 anymore, but IE6 still accounts for more than 5% of my site traffic. Multiple IE basically helps me test both IE6 and IE8 from the same machine. However, there is still the IE7 gap, plus Multiple IE, which is no longer being updated or maintained, can produce some glitchy behavior.
It turns out, though, Microsoft offers some tools to help test their legacy browsers.
Microsoft offers several Windows virtual hard disks (VHDs) that can be loaded using Virtual PC. If you run IE8, you can download VHDs that include IE6 and IE7, and run them through a virtual environment.
Two drawbacks compared to Multiple IE: the VHD files are huge (some are 800+ MB), and in my experience, the virtual machines run hopelessly slowly. (The latter may be due to my environment or my Virtual PC settings.) However, it does allow you to test web pages on fully-functional versions of IE6, IE7, and IE8 without maintaining separate testing machines.
I’m running VMWare Fusion on Mac OSX (Leopard), which allows me to test Firefox and Safari on the Mac, as well as the PC versions of the browsers through a Windows VM. I could presumably create separate VMs for IE6 and IE7, although that may not be kosher with Microsoft’s licensing for my copy of Windows XP. It is frankly easier, however, to run Virtual PC through my Windows VM, than to install Windows XP twice more over.