I haven't dl'ed the demo, so I'm mostly shooting in the dark, but here's an idea: from the docs that the demo comes with (if any :P), or from the forums, are there any command line parameters that you can run the game with ? Sometimes these let you run with reduced functionality (low res from the start, no sound, etc), and that might at least let you start the game.

Alternatively, you can try to temporarily disable/deactivate any firewalls/anti-virus/anti-spyware software that you may have running - sometimes they don't let software write to the registry (although they should ask you) or do "dirty" things.
Normally, you should get warnings/errors, but I've personally had many cases where various games (or applications) wouldn't install or run because of my antivirus/firewall/whatever.

good luck.