For the current user:

System.Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles ));

For all users

System.Environment.GetFolderPath(Environment.SpecialFolder.CommonProgramFiles ));

32 bit executable under windows 64 bit for the current user

 System.Environment.GetFolderPath(Environment.SpecialFolder.ProgramFilesX86 ));

For the current user

System.Environment.GetFolderPath(Environment.SpecialFolder.CommonProgramFilesX86 ));