Sorry but this isn't clear for me. If you create a new user who"s nam is openerp at that time you give something. If you do not remember you can change it by root. (Also in Ubuntu you can change it as sudo) If you have just ine user > openerp, you need to hack your system (Ubuntu) by this way.:http://www.psychocats.net/ubuntu/resetpassword

I am talking very clear: I am installing openerp on ubuntu and using debian package. Before the installation of the debian package, there was not username called "openerp", after the installation done (I did this using debian package), a username "openerp" has been created but I have not been prompted to give it the password ! It seems the installer create the username and the password. What is this password that is created by the installer?

Please note you can change the system password, without any issue. One what is important the openerp user PostgreSQL pass. Sorry for misunderstanding, I usually install Openerp by manually.... to change the pass just type in terminal: sudo passwd openerp