In Windows XP Home, you can login as Administrator [built-in] only in Safe Mode. To show the Administrator login in Welcome Screen [XP Professional], try this:
Start the Registry Editor [Regedit.exe]
Go to HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \ Winlogon \ SpecialAccounts \ UserList \
Right-click an empty space in the right pane and select New > DWORD Value
Name the new value Administrator
Double-click this new value, and enter 1 as it's Value data
Close the registry editor
