@prince-xyryl I do not believe the built-in administrator account can be deleted, but you can disable it.
I am assuming you have something similar to this in your unattend file, correct?<AutoLogon> <Password> <Value>notmypw</Value> <PlainText>true</PlainText> </Password> <LogonCount>3</LogonCount> <Enabled>true</Enabled> <Username>administrator</Username> </AutoLogon>
If you remove that block when you run sysprep with the generalize switch it should not enable the administrator account. Just make sure you create your other account before hand.
If you need the administrator account after sysprep you can disable it with this command: net user administrator /active:no
More information: https://technet.microsoft.com/en-us/library/Cc766343(v=WS.10).aspx