DaveyDarko: I think mine did the same thing the when I tried, Witcher 2 worked fine but Witcher gave me the error, but if you look at the right side of the page there is a link:

http://thewitcher.com/backup/

that will give you another way to show you own the game on Steam, and then you will get a key emailed in a few days. That's how it worked for me at least.