How to change user account types vary depending on if you are on a domain or a workgroup. The steps outlined here should help you manage your user accounts - http://windows.microsoft.com/en-us/windows7/Change-a-users-account-type
Microsoft Windows Outreach
Thank you for your quick answer. The workstation is in a workgroup.
For some reasons the system is been corrupted, and I do not have any account with admin right including the admin hidden account.