An administrator account is the at the hierarchy to manage all the activities for a system. Since the local administrator account is disabled by default, so in order to enable it, we need user who is the part of default administrator group. He can easily enable built-in administrator account using the steps illustrated below:
Activate Administrator Account For Windows 8.1 In WorkGroup Mode
1. Press
Windows Key +
R combination, type
lusrmgr.msc in
Run dialog box and hit Enter to open the
Local Users and Groups snap-in.
2. In the Local Users and Groups window, click
Users from the left pane, then
right-click the
Administrator in the center pane. Select
Properties.

3. In the Administrator Properties window,
uncheck the option
Account is disabled. Click
Apply followed by
OK.

4. Again right-click on
Administrator and select
Set Password in following window:

5. Now click
Proceed in the following window:

6. Finally,
create a strong password in the following window. Click
OK.
Click here for Tips for creating a strong password

In this way, the built-in administrator account is bought out into action. If you are
NOT the administrator, You should notify the administrator about the change you’ve made to the system.
This procedure also works in Windows 8.via:twc & microsoft