Hi Searcher,
I've looked into this further, and there are other requirements.
Besides the Rights Management Add-on mentioned below, the user will
need the Windows Rights Management Client
(
http://www.microsoft.com/downloads/details.aspx?familyid=3115A374-116D-4A6F-BEB2-D6EB6FA66EEC),
a link to which is on the add-on's download page. The client must be
installed first. Discouragingly, the two downloads total about 8 MB.
Once the recipient is set up with the software, the author of the
protected document has to take an extra step to enable the add-on to
read it. In Word 2003, in the Permission dialog that invokes IRM, you
have to click the More Options button and then check the box for
"Allow users with earlier versions of Office to read with browsers
supporting Information Rights Management". This causes Word to save
additional information in the file for IRM.
Finally, when the Word 2000 user gets the file and can open it, it
appears as a web page in Explorer, not as a page-formatted document.
If you need the latter, this won't work for you at all. Can you use
PDF instead?