Here is some helpful information from the MS IEBlog about how to run IE 6 and 7 on the same machine.
IEBlog : IE6 and IE7 Running on a Single 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.
C. David Gammel, CAE 
This has always been such a huge draw back, even when they went from 5, to 5.5 to 6.0. It was always so annoying that the application just overwrote the previous version. I guess this is what you have when you have it so tightly bound to the OS. I say, lovely planning from the get go. Whoever made the decision to integrate it from the get go is a genius…I mean this simple decision is the core of 80% of their security issues alone.
Good point! They should have identified it as a problem way back at 5.5 and provided a solution. I guess that’s why BrowserCam has a sustainable business model.