I had the same problem. I fixed it using the Regedit fix talked about here
http://www.gog.com/en/forum/the_witcher_2/downloadable_content/post31 You can open Regedit by opening up the start menu, clicking "Run", and typing in regedit. Then just delete the "Downloads" folder talked about in that thread. It'll also remove any record of having downloaded the Troll Trouble DLC, but you can easily redownload that and go through the process again.