Could you try to install using the appropriate stand alone installer which you can find here?
Thank you very much Chris! Finally i coud install flash player. Please, can you explain me why? And what about Adobe Reader ( it acts in the same way)?
The default installer (install helper) sometimes acts up, e.g. when you have a slow connection, or for many other reasons.
In this case, the offline installer will succeed most of the time.
You can find the offline installer for Adobe Reader at http://get.adobe.com/reader/enterprise/
Ok, thanks to all. I solved the problems