Right click My Computer, then go to Manage. Expand Local Users and Group, then Local Users. You can add membership to the admin group to any user by right clicking and adding them in the Member Of tab.
Why are you doing it this way?? This is like making a peanutbutter sandwich but trying to do it without using a plate and not setting anything down.
Start->Control Panel->User Accounts.
In here you click on the user name that you want to set to admin and then you'll get a list of options. Select "Change account type" and then select the radio button that says "Administrator" and save changes.