First, go to Control Panel -> Programs and Features (or, on Windows XP, Add/Remove Programs), and uninstall these components:
.NET Framework 4 Extended
.NET Framework 4 Client Profile
Just to make sure, use the .NET Framework Cleanup Tool, which can be found here, and uninstall .NET Framework 4.
Afterwards, restart your computer, then download and install the .NET Framework 4, which can be found here.
You may need to change some options in The Witcher 2's launcher in order for this fix to take effect. If this doesn't help, contact GOG Support.