After reading a blog post by Shelley I downloaded Virtual PC 2004 and a Virtual PC image of Windows XP with IE6 installed. The idea is that you can use it to test IE6 on your machine when you've already got IE7 installed. It works on Windows 2000, by the way, even though it says it doesn't. When you download the image you have to run the executable to unpack itself on XP, but from then on it all works on 2000.
Just for fun I then decided to upgrade IE on the Virtual PC to IE7. When it got to verifying whether the copy of Windows on the Virtual PC was genuine it concluded it wasn't. Click on the picture for proof.