Originally Posted by Jigoku
Steam version: HKEY_CURRENT_USER\Software\CD Projekt RED\The Witcher 2
Argh! That was the other one I was trying to find but lost the page.
Yeah, there's a bunch of different locations people have found it in with different versions (of the game) on Steam, and different OSs, so it's been somewhat troublesome. At least they've got the webpage up to find it on your profile if you have no luck at all.
EDIT: I'm guessing, the easiest way to find it is to just fire up Regedit, hit CTRL-F and enter The Witcher 2
- in fact I'm surprised none of the links I was reading just said that.
It might take a few F3s (search again) to find the correct one, but likely the first you find will have the serial.