To change the default Pc administrator name

When installing windows in my computer the installation boy gave a odd name "user" which I want to change and at that I want to give my name. I tried to change the user profile manly times it was changed but it also shows the previous name "user" and after changing I couldn't get some of the permission such as to open folder option using shortcut menu asking for a permission from the administrator but previous one was not asking for the permission to do this simple tasks. Overall I want to remove that previous admin.. name and give my name a new admin... name. Please clarify how to do it. Thank you,