With the release of IE7 many web developers were faced with a need to test their applications on the different versions of IE, but had no means to do so since only one IE can run on Windows. Now there were different hacks available around it, but in most instances you ended up using portion of IE7 libs for IE6 emulation and thereby not getting quite the same behavior.
Today on
IE blog a much better solution was offered by Microsoft (kudos guys). Basically they've allowed Windows owners (after genuine advantage check, which now can be done via Firefox as well) to download WinXP SP2 image with IE6 and run via a free download of "Virtual PC 2004". This means you can safely upgrade you WinXP box to IE7 and run IE6 via an image, thus giving you 2 versions of IE on the same machine this minimum amount of hassle.