http://www.gog.com/en/forum/the_witcher_2/cant_install_patch_1_2 This should be helpful.
The near-fail-safe method is:
1. Get patches 1.1 (
here ) and 1.2 (
here). Make sure you download the right version of 1.1 for your game version (retail, gog, regional restrictions...)
2. Un-install game, delete all remaining folders and files except your savegames (this includes the Documents\Witcher 2\Downloads folder!)
3. Reboot, disconnect internet
4. Install the game by starting the setup.exe with 'run as admin'. Do NOT start the game afterwards.
5. Install Patch 1.1 using 'run as admin'
6. Install Patch 1.2 using 'run as admin'
There might be a quicker or easier solution, but if you don't want to mess around long trying to find out what exactly the problem is, this should work.