FYI: When I selected "Add to GOG.COM downloder" I received the message "GOGDOWNLOADER://The_Witcher_/ is not a valid win32application." When I "X"'d out of that message another one popped up reading, "Unable to open this helper application for gogdownloader://the_witcher_2/. The protocol specified in the address is not valid. Make sure the address is correct and try again."
I removed the program (gogdownloader) and all associated files. Turned off all anti-virus and re-installed/ tried/ and uninstalled several (many)times. Turned UAC off. I updated all my hardware. Everything else was fine. After hours of frustration I decided to share this fix. When downloading the gogdownloader install it to the Windows "Program File" instead of the default "Download" file chosen by Windows 7 (and maybe Vista as well). I downloaded to the Program file instead and immediately the problem was resolved. Just wanted to help others.