There's a new guide on About on installing two versions of IE on your PC:
Like most Web designers, I need to test my Web designs on as many versions of as many browsers as I can, so that they display correctly for all my customers. Internet Explorer makes that very difficult because IE is built into the Windows OS - and so even if you use a hack, it doesn't display the same as on a stand-alone system.
But it is possible to use a Virtual PC to run IE 6 on the same machine that you have IE 7 on natively.
While it's not easy to install (there are 19 steps) and there are some gotchas (the hard drive image Microsoft provides expires on April 1, 2007), this is still a less expensive way to test your creations on IE 6 while taking advantage of all the changes in IE 7.