With the release of IE7 many web developers (like me) were faced with a need to test their applications on different versions of IE, but it was impossible for them since there is only one version of IE can be installed on Windows.
But today, as announced by IE Blog, Microsoft has made Virtual PC (VPC) 2004 available for download which contains a pre-activated Windows XP SP2, IE6 and the IE7 Readiness Toolkit to help facilitate developer to test their application on different version of IE (6 and 7).
Now you can install IE7 on your main machine for development, and get all the advantages of IE7, like the RSS platform, native XMLHTTP stack, and improved security, while still running IE6 simultaneously in the VPC on the same computer. Most importantly, you don’t even have to buy an additional Windows license. The VPC image runs in a virtual machine that offers all of the functionality of a full IE6 installation without giving it any access to its host machine’s hard drive, registry, etc. You can make as many modifications as you want to the virtual machine without affecting your host installation at all.
Get more information on Virtual PC 2004 here.