http://msdn2.microsoft.com/en-us/library/ms995853.aspx
CSIDL_COMMON_APPDATA
This folder should be used for application data that is not user specific. For example, an application may store a spell check dictionary, a database of clip-art or a log file in the CSIDL_COMMON_APPDATA folder. This information will not roam and is available to anyone using the computer. By default, this location is read-only for normal (non-admin, non-power) Users. If an application requires normal Users to have write access to an application specific subdirectory of CSIDL_COMMON_APPDATA, then the application must explicitly modify the security on that sub-directory during application setup.
This folder should be used for application data that is not user specific. For example, an application may store a spell check dictionary, a database of clip-art or a log file in the CSIDL_COMMON_APPDATA folder. This information will not roam and is available to anyone using the computer. By default, this location is read-only for normal (non-admin, non-power) Users. If an application requires normal Users to have write access to an application specific subdirectory of CSIDL_COMMON_APPDATA, then the application must explicitly modify the security on that sub-directory during application setup.