Hi, in some cases if you do not belong to the administrator group, you would not be able to make changes like altering account type. Under such circumstance, you should activate the true Administrator account hidden by default and log in with the Administrator account.
Click Start button, then type "cmd" in the Run dialog.
Open the Command Prompt.
Type "net user administrator /active:yes" and press Enter key. Then type "exit" and press Enter key.
then you can restart the machine and log on with the very Administrator account.
You can follow detailed guides posted in http://blog.doofix.com/how-to-set-yourself-up-as-the-administrator-in-windows/,
then take steps to change user account type with the help of http://blog.doofix.com/how-to-make-a-user-account-the-administrator-of-the-windows/
Hope this helps!