If you previously purchased The Witcher 2, you can get a free upgrade to the DRM-Free Witcher 2 Enchanced Edition at GOG.COM
If you have a boxed copy or purchased it online (from g2play or similar sites), enter your serial number in the box. If you purchased it on Steam, you'll need to use Regedit to find your key. Depending on your version of Windows, it'll be in one of a few places.
Check the following:
HKEY_LOCAL_MACHINE\SOFTWARE\CD Projekt Red\The Witcher 2
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\CD Projekt Red\The Witcher 2
If you don't have it installed, or have the 1.2 update, you might not be able to find a serial, in which case go to this page: The Witcher 2: Assassins of Kings Enhanced Edition
There, you can provide proof of purchase, or if you don't have that, they can check your Steam account (you'll need to set profile to public) automatically and email you your key.
PS: Although it doesn't say anything on the GOG page, the official site says this offer ends on May 17th
, so get in while you can! It's always worth having DRM-free copies of games.
Especially with all the bonus content you get from GOG.