Brian is writing on his blog that Microsoft offers a free Windows XP including Internet Explorer 6 as a VPC image to test web sites with IE6:
Many of you have asked how to run IE6 and IE7 in a side by side environment. As Chris Wilson blogged about early this year, it’s unfortunately not so easy to do. There are workarounds, but they are unsupported and don’t necessarily work the same way as IE6 or IE7 would work when installed properly. As Chris said, the best way to use multiple versions of IE on one machine is via virtualization.