Do not hide all your admin accounts!!!
To find what the actual names of accounts are open computer management. Under Computer Management -> System Tools, select the item Local Users and Groups. Double click Users. the value of the first column, "Name".
Open regedit and find
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Create 2 subkeys (SpecialAccounts and UserList) so the path will look like this.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList
Create a new DWORD value in the UserList subkey. Use the login name which you noted earlier in Local Users and Groups as the name of the new value you just created. Do not modify its default value, leave it at 0.
To show that account again, just delete the DWORD value you created earlier under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList Registry key.
Do not hide all your admin accounts!!!