As explained in this article, the applications you create with one version of the framework, can target only that version or both, according to the settings you put in the appname.config file. (Where appname is the full name of your executable file including the extension, such as myapp.exe, so the config file would be myapp.exe.config). To make VS.NET 2003 automatically create and properly set this file for you, open the Project Properties dialog, select the Common Properties / Build sheet, click the Change