Like Windows ME, Windows 98, Windows NT and others. I want to test and compare operating systems for my satisfaction of learning new things.
If you have a MSDN paid subscription, you have an access to various downloads, and especially old (and very old like Windows 3.11) operating system ISO (CD and DVD) along with the required licensing information.
Try Microsoft's Internet Explorer Application Compatibility VPC Images. You can't get really old OSes, but you can use them to legally play with Windows XP and Vista.
They do expire after some time, but you can always download the new versions and continue testing/playing.
You can't. Although these operating systems are old, unsupported and no longer for sale, they are still owned by Microsoft and therefore are still illegal to distribute for free (Unless you are Microsoft).
Beta Archive has lots of old software, but you need to make 10 contributing posts on the forum to get FTP access.