Hi Hicham,
The username will be reset during this process :)
Recovering the OS Administrator Password
If you lose the Administrator password and cannot access the system, use the following procedure to reset the Administrator password.
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Note During this procedure, you will be required to remove and then insert a valid CD or DVD in the disk drive to prove that you have physical access to the system.
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Procedure
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Step 1 Log in to the system with the following username and password:
•Username: pwrecovery
•Password: pwreset
The Welcome to admin password reset window displays.
Step 2 Press any key to continue.
Step 3 If you have a CD or DVD in the disk drive, remove it now.
Step 4 Press any key to continue.
The system tests to ensure that you have removed the CD or DVD from the disk drive.
Step 5 Insert a valid CD or DVD into the disk drive.
The system tests to ensure that you have inserted the disk.
Step 6 After the system verifies that you have inserted the disk, you get prompted to enter a new Administrator password.
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Note The system resets the Administrator username to admin. If you want to set up a different Administrator username and password, use the CLI command set password. For more information, see Appendix A, "Command Line Interface."
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Step 7 Reenter the new password.
The system checks the new password for strength. If the password does not contain enough different characters, you get prompted to enter a new password.
Step 8 After the system verifies the strength of the new password, the password gets reset, and you get prompted to press any key to exit the password reset utility.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/cucos/6_1_1/cucos/iptpch2.html#wp1040303
Also, try this great, great post from Michael @ Cisco;
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=IP%20Telephony&topicID=.ee6c829&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.2cc0bd9a
Hope this helps!
Rob
