Finally I have made a working code; if It may help somebody else here it is:

Qt Code: 
  1. MainWindow::MainWindow()
  2. {
  3. 	...
  4. 	http = new QHttp(this);
  5. 	connect(http, SIGNAL(requestFinished(int, bool)), this, SLOT(httpRequestFinished(int, bool)));
  6. 	connect(http, SIGNAL(responseHeaderReceived(const QHttpResponseHeader &)), this, SLOT(readResponseHeader(const QHttpResponseHeader &)));
  7.  
  8. 	file = 0;
  9. 	downloadFile();
  10. }
  11.  
  12. MainWindow::~MainWindow()
  13. {
  14. 	if(file) {
  15. 		delete file;
  16. 		file = 0;
  17. 	}
  18. }
  19.  
  20. void MainWindow::downloadFile()
  21. {
  22. 	if(file) {
  23. 		delete file;
  24. 		file = 0;
  25. 	}
  26.  
  27. 	QUrl url("http://localhost/sito/lastrelease.xml");
  28.  
  29. 	file = new QTemporaryFile;
  30. 	if (!file->open()) {
  31. 		statusBar()->showMessage(tr("Impossible to save the file %1: %2.").arg(file->fileName()).arg(file->errorString()));
  32. 		delete file;
  33. 		file = 0;
  34. 		return;
  35. 	}
  36.  
  37. 	http->setHost(url.host(), url.port(80));
  38.  
  39. 	httpRequestAborted = false;
  40. 	httpGetId = http->get(url.path(), file);
  41. }
  42.  
  43. void MainWindow::httpRequestFinished(int requestId, bool error)
  44. {
  45. 	if (requestId != httpGetId)
  46. 		return;
  47. 	if (httpRequestAborted) {
  48. 		if(file) {
  49. 			file->close();
  50. 			file->remove();
  51. 		}
  52. 		return;
  53. 	}
  54.  
  55. 	if (requestId != httpGetId)
  56. 		return;
  57.  
  58. 	file->close();
  59.  
  60. 	if (error) {
  61. 		file->remove();
  62. 		statusBar()->showMessage(tr("Download failed: %1.").arg(http->errorString()));
  63. 	} else {
  64. 		statusBar()->showMessage(tr("lastrelease downloaded"));
  65. 	}
  66. }
  67.  
  68. void MainWindow::readResponseHeader(const QHttpResponseHeader &responseHeader)
  69. {
  70. 	if (responseHeader.statusCode() != 200) {
  71. 		statusBar()->showMessage(tr("Download failed: %1.").arg(responseHeader.reasonPhrase()));
  72. 		httpRequestAborted = true;
  73. 		http->abort();
  74. 		return;
  75. 	}
  76. }
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Also the xml part of the work is ok. I have to decide where store the current versions of application/database: for application probably is better in the code for simple retrieving; for database I don't know if is better to store it in a table or in the name of the db file (db-20070115 for example). Perhaps the first solution is simpler. What do you say?

Thanks