uses Registry;

procedure TForm1.Button1Click(Sender: TObject);
var
reg: TRegistry;
begin
reg := TRegistry.Create;
with reg do
begin
RootKey := HKEY_LOCAL_MACHINE;
OpenKey('SoftwareMicrosoftWindowsCurrentVersionSETUP', false);
ShowMessage(reg.ReadString('SourcePath'));
CloseKey;
free;
end;
end;