Some of the antivirus programs (e. g. McAfee, Ikarus, Norton) may detect a virus in the witcher2.exe file. It's a false positive, the file is not harmful to the system, nor to the user's computer, in any way. CD Projekt RED has contacted antivirus manfucaturers in order to dispose of the problem.
Some of the programs may yield a false positive if their threat definitions were not updated. Please make sure that you installed all the latest updates for the antivirus.
If despite the installation of the newest updates the problem still occurs please temporarily disable your antivirus software (for the duration of installation and the download of the game updates).