You may wanna hold off downloading this 1.1 GB... looks like someone didn't test this 'alpha' release: while the unrar runs fine, the setup errors out with "The PhysX runtime distributable package was not installed succesfully. Setup cannot continue."
Edit: Ah, found a bypass: After the unrar cancel the setup, go into the PhysX subdirectory and run that installer. Now EASetup.exe in the main directory continues and installs the demo. Phew.