If you need to run IE 6 and IE 7 on the same system for some reason (testing?), what are you supposed to do?
One solution is the "Multiple IE installer" from TredoSoft. I haven't tried it, but it is certainly a novel approach. Looks like you can run IE3, IE4.01, IE5, IE5.5, and IE6 with it.
TredoSoft also provides instructions and a utility that allows you to run IE7 side-by-side with IE6. Again, I haven't tried it, but it is good to know that it is available.
When I need to test different versions of the browser, I have as of late been relying on VPC images of Windows XP with IE6 and IE7 that have been provided by the IE team. The VPC images run with the free Virtual PC 2007. They expire on 2007-08-17, for various reasons, but the IE team has in the past provided a "refresh" for the IE6 VPC image when it last expired, and the expectation is that this will continue. The expiration-refresh cycle allows for control over how "old" an image is allowed to get - refreshes have the latest security patches installed so people aren't left running ancient and (overly) vulnerable VPC images.
The images won't pass WGA for obvious reasons but this hasn't affected my ability to test with them.
I tend to customize my IE settings pretty severely, so it is nice to be able to test with "stock" / "virgin" installs of IE. One might argue that using the VPC images doesn't necessarily mean one has two different versions of IE on the same system - that the introduction of a VM means another "system" is involved. While this is true, the VM solution does prevent one from needing to have multiple physical machines around just for the sake of testing. And, while the TredoSoft solution may work, I can't help but think that the possbility exists for the solution itself to be a potential cause of problems.