If you develop for the web (and you care about your users) then you must test your applications in the 4 - 5 "top tier" browsers, which include Internet Explorer, FireFox, Safari, Opera, and Netscape. Furthermore, you usually need to test your applications in a few different versions of the top tier browsers to make sure you know how your site will behave when users of old browsers come a knockin'. This is a straight forward task for all browsers but one: Internet Explorer.
Microsoft officially introduced Internet Explorer 7 to the world last month and officially made developers lives harder. You cannot run IE6 on a computer that has IE7 installed. While there are hacks out there that allow you to "trick" the OS into running a special IE7 build on an IE6 computer, they are not supported by Microsoft and they do not accurately simulate your users' IE7 experience. It is unclear why Microsoft thought developers would not need to continue IE6 testing for years to come (and thus provide no support for side-by-side testing), but they have seen the error of their way and offered a new option.
Last week, the IEBlog announced that Microsoft would be making Virtual PC 2004 bundled with a pre-activated copy of Windows XP SP2 and IE6 a free download. That's right, you can now do Virtual PC testing of you web apps without the need to purchase an additional Windows XP license (the requirement until last week). This is huge news and a very generous (albeit necessary) peace offering from Microsoft to help developers test applications in older versions of IE. It is possible that VPC images with older versions of IE (like IE5.5, IE5, etc.) may become available, but for now it's just IE6.