You can make a call to the Windows API function 
GetWindowsDirectory().
If you want the system directory, simply call the Windows API function 
GetSystemDirectory().
Example:
{$IFNDEF WIN32}
 const MAX_PATH =  44;
{$ENDIF}
procedure TForm .Button Click(Sender: TObject);
var
  a : Array[0..MAX_PATH] of char;
begin
  GetWindowsDirectory(a, sizeof(a));
  ShowMessage(StrPas(a));
  GetSystemDirectory(a, sizeof(a));
  ShowMessage(StrPas(a));
end;