DirectoryInfo dirinfo = new DirectoryInfo(@"C:\Test");
System.Security.AccessControl.DirectorySecurity dirsecurity = dirinfo.GetAccessControl();
dirsecurity.AddAccessRule(new System.Security.AccessControl.FileSystemAccessRule("IIS_WPG", 
    System.Security.AccessControl.FileSystemRights.FullControl, System.Security.AccessControl.InheritanceFlags.None, 
    System.Security.AccessControl.PropagationFlags.None, 
    System.Security.AccessControl.AccessControlType.Allow));

dirinfo.SetAccessControl(dirsecurity);