Vista - Sysprep and Administrator account

Here is a scenarion, I have deleted all user account and enable the
Administrator account, and set it to auto logon, so that it can boot directly
into the account and run some automated tests.. Now when i sysprep such a
machine and create a image, After deploying the image with a unattend.xml,
it disable the Administrator account. So i left with a unusable system with
no valid account to login.

Now here is my question
1. Is this a bug in Vista? Or this is not a valid scenario?
2. After getting stuck, Is there a way to activate the Administrator account
by booting through Windows PE?

Vista Version : 5600 RC1

Waiting to hear from you, please help me out

Regards
/Moyukh