Here is some helpful information from the MS IEBlog about how to run IE 6 and 7 on the same machine.
This is really important for any web developer or designer since IE6 will be with us for quite a long time to come. The downside here is that it isn’t possible to run both browsers within the same instance of the operating system, which is why MS is offering a free virtual server with which to run 6 in parallel. Not ideal.
I’m surprised that MS didn’t foresee this need before they launched 7.