Edit 3: Solved. I really should see if there's a similar problem first. Ugh...

So, I just installed the game and when I'd like to start it, I receive an error message.
Appartly, something called "d3dx9_35.dll" can't be found.
I also receive the advise to install the game again. But after uninstalling and reinstalling, I still get the error message.
And I have no clue what this means. Uhm... help?

Oh, yes, I'm using Windows XP, if that's important.

Edit2: Ugh, I'm in Germany, it's late and obviously I'm too stupid to browse through this forum.
Just found this topic: http://www.gog.com/forum/the_witcher/the_game_wont_start_because_d3dx9_25_dll_is_missing
Trying to upgrade right now. Hope that does the trick. Or else, I'm going to scream. Or something.

Thanks for the kind words.
I hope you don't mind, if I put your response as answer to my question. To make it official, that the problem has been solved. ;)
(If that's not against the rules...)