To set up the real admin account see this tutorial: Built-in Administrator Account - Enable or Disable
In Windows accounts in the Administrative group are similar to Windows NT Server Operators Group. You can register ActiveX Controls, install and remove programs, but you can't delete system files. It's not really the full-powered Administrator account.
The easiest is probably right click on the installer and select Run As Administrator. If it still won't let you install then you might have to create the Admin account as shown in the tutorials.