For my latest Joomla! site, I have set up a Windows XP Virtual Machine (VM), using the VirtualBox Open Source Edition. Trouble is, once I'd installed Internet Explorer 7, I couldn't get a standalone copy of Internet Explorer 6 working.
So, I created a new WinXP VM and then cloned it using these instructions from the VirtualBox forums. Now, I have standalone IE5.5 (from the browser archive at evolt.org) and IE6 on one VM, IE7 on another and IE8 on a third.
And I can test away to my heart's content for those poor unfortunates locked into a 'World with Windows but no way out'™ ;)