The demo version of Altium Designer is too big to download. There is no legal place to download the demo on the web. Altium will send you a demo on disks and a 30 day license if you contact one of their sales offices. You can get the contact information on the Altium website.
The current version of Altium Designer is version 6. AD2004 is no longer available from Altium in any form, including the demo.