I’ve written before about using Microsoft’s IE Application Compatibility Images with VirtualBox. These images provided a very convenient, and free, way to setup testing environments in all versions of Internet Explorer.
The images, since they are free copies of Windows, expire after a time, requiring you to download a new image. The latest images provided by Redmond, require activation if you use VirtualBox, or probably any other virtualization environment instead of VirtualPC, which is windows-only. Whether or not this is intentional is hard to say, but I don’t think that it is.
If you have the old images, it seems like you can keep using them. The only limitation I’ve run into is that the machine shuts down after an hour. For web-development work, that’s been sufficient time for me to test and fix web broswer rendering bugs.