Of the various methods I have tried, using spoon.net is by far the quickest and simplest way to run older browser versions (as well as many different browsers). It is simple to get up and running. It is also easy to debug web applications running on localhost (using IIS or Visual Studio's development web server). You will need to pay a small fee (~ $5/month), but that is well worth the time it saves.
(2) Windows XP Mode and Virtual PC
The following article explains how to use Windows XP Mode and Virtual PC to run IE6 and IE7:
It took an hour or two to get everything installed and configured, but it was fairly straight forward. I created a third XP virtual machine for IE8; so I can run IE6, IE7, and IE8 on my development machine. A couple of things to consider:
(a) If you want to run more than one of the virtual machines simultaneously, you should rename each machine (from within the XP guest OS) to avoid NetBIOS naming conflicts
(b) Since the browser is running in a virtual guest OS, it is not straight forward to access a web application running on localhost of the host OS. I am sure it can be done, but I have not yet take the time to figure it out.