As many know testing different versions of IE can be a pain, there’s no supported way to run two versions side by side (people have done it, but Microsoft don’t recommend it and can’t guarantee accuracy of results). Their reccomended way was to run another instance of XP in Virtual PC, Microsoft have made Virtual PC free as they’re competing with VMware in the virtualisation market, however XP is not free and so to legally use this solution you needed another XP licence. A rather expensive solution just to test one browser.
Microsoft have now decided to make it free and slightly easier, they’re producing time limited images of XP with IE6 installed for Virtual PC, this means that you and legally run XP within a virtual machine for IE6 testing purposes while having IE7, Firefox, Opera, etc installed on your primary machine. The draw back is that this image is time bombed and will expire in April 2007, however, Microsoft plan to keep releasing updates to this image. I guess the idea is to make it too inconvenient to use this image for anything other than browser testing, if the image did not expire then people could use it for whatever they wanted and in effect get a free XP licence. So it’s not 100% perfect solution, but it’s a major improvement so it will make the lives easier of many web developers and testers.