Hi georgebridge, welcome to TSF..

you should find the following helpful:
http://www.maximumpcguides.com/logon-windows-vista-as-administrator/

 

Hi. . .

To activate the Vista hidden admin account, do this -

START | type cmd.exe into the start search box | right-click on cmd.exe above under programs - select run as admin. Then type this into the black cmd prompt screen:

Net user administrator /active:yes

Now logoff your account and the user account named Administrator will be waiting for you.

Be cautious with that account... it is your last line of defense.

Regards. . .

jcgriff2

.

 

Another way is to run user management.
Click start, run, and type lusrmgmt.msc and press enter. Double click the Administrator account, then uncheck the box 'account is disabled'.