From here:
FOLDERID_ProgramData / System.Environment.SpecialFolder.CommonApplicationData
The user would never want to browse here in Explorer, and settings changed here should affect every user on the machine. The default location is %systemdrive%\ProgramData, which is a hidden folder, on an installation of Windows Vista. You'll want to create your directory and set the ACLs you need at install time.
So, just use %PROGRAMDATA%
, or better still:
Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData)