I registered my normal edition DVD copy on this site yesterday and downloaded my Enhanced Edition. I installed it, now I'm clicking on the icon, press ''Launch Game'', and I receive this Error code with a classic Windows error message : 0xc0000142.

My normal edition is still installed, and it works.

what am I doing wrong ?

Sounds very familiar to this issue:

http://www.gog.com/en/forum/the_witcher_2/error_0xc0000005

Have you contacted support?