If you own a copy of The Witcher 2 (doesn't matter where you bought it) you can download a free DRM-free copy of The Witcher 2 Enhanced Edition from GOG.com.
Just go tohttp://www.gog.com/en/witcher2/backup/
and enter your product key.
If you don't have a product key (eg. you bought it on Steam) then you can request a key by following the steps on:http://www.thewitcher.com/backup/