uses Registry;
procedure TForm1.Button1Click(Sender: TObject);
var
    reg: TRegistry;
begin
    reg := TRegistry.Create;
    reg.RootKey := HKEY_LOCAL_MACHINE;
    reg.OpenKey('Software\Microsoft\Windows\CurrentVersion\SETUP',false);
    ShowMessage(reg.ReadString('SourcePath'));
    reg.CloseKey;
    reg.free;
end;