Did you register the software when it was originally installed? If you did you can find your serial number in your account at Adobe.com, and you can download an installer from the links at the bottom of Download and Installation Help
Hello, Peter and thank you for your answer.
No, unfortunatly I did'nt... and i try to contact an hotline commercial assitance, without success. No answer by telephone for my product !
Maybe by tchat ? I could have any answer you think ?
You can try chat, but I think unless you registered the software there is no way anyone can help you recover a lost serial number for CS5.
There's a script at http://indesignsecrets.com/locating-your-indesign-serial-number.php that might be of use if ID is still installed, but I'm not sure it gives enough digits to help you.