Administrator account - will not let me create

I need to install some software on a user's machine. We have two machines w/
Vista and i got one to accept the Administrator account. But on this other
machine it is being very frustrating. I did the workaround on the first
machine editing the registery by adding the special accounts key in the
HKLM/Software/microsoft/windowsNT/CurrentVersion/Special accounts/userlist
and it worked fine w/ no hitches but this other PC......first i tried to
login as the administrator but it tells me that the username and password are
incorrect.....WHAT!?!?!?!?!?!?!?!??!!??!?!!?!?!?.. ...thinking i misspelled I
typed again.....and again.....and now it locked the
account.....WHY!!!!!!!!!!!!!!!!!

So i log in (I have admin rights) but it will not let me add the
Administrator to the user list because it says it is already added but it
does not show up in the user list.....so i figure i'll run the regedit and
add the key.....nope...tells me i do not have permission to do that.....

Help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!