I have just done a fresh install of Windows VISTA Business x64 and have created 2 user accounts

one is standard, the other is an admin account

I have hidden the admin account using HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserLis t

How do I login to the Admin account now?

You can still log in using this method:

Press start->run
Type: secpol.msc
Press enter
Expand local policies
Click Security Options
Double-click "Interactive Login: Do not display last user name"
Check Enabled
Click OK
With both of these settings in place, the same functionality of the old-style "Press CTRL + ALT + DEL to login" window is achieved.

Vista How to disable welcome screen and fast user switching?

It prompts for an administrator password, but there is no username and no password field

Try typing in the admin account password from the hidden acct. When you activate this policy what it does is force you to type in a username AND password when you log in to Windows. You won't get a list of any user accounts on the Welcome Screen, but you can now type in the username of the hidden account.

still can't get into it
it says when i run the secpol.msc that i don't have permission to run it