Ever since IE7 came out developers have had the problem of whether they should upgrade or not. If they chose to upgrade, they wouldn’t be able to test their sites in IE6 any longer as installing IE7 would uninstall IE6. There are 3rd party hacks that allow both versions to be installed at the same time, but they will not simulate a true IE6 / IE7 environment.
In recognition of this problem, Microsoft has put out a free Virtual PC 2004 (VPC itself has also been made a free download) image containing a pre-activated Windows XP SP2 installation as well as IE6 and the IE7 readiness toolkit. The image will self destruct on the 1st of april 2007, there’ll probably be another solution out by then.