Where can I download previous versions of Microsoft Operating Systems?

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.