The built-in Administrator account is hidden from the "Welcome Screen" when a user account with Administrator privileges exists and enabled.
In Windows XP Home Edition, you can login as built-in Administrator in Safe Mode only. For XP Professional, press CTRL+ALT+DEL twice and input your Administrator password in the classic logon window that appears. however, i've noticed even that didn't always work.
To have a user or Administrator account display in the Welcome Screen, edit the registry
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList
find the user (or the "administrator" account) you want displayed in the login screen, and set the value to REG_DWORD = 1. (you can add "administrator" if it's not already there).
NOTE for Windows XP Home Edition users: While you can configure Windows XP Home Edition to show Administrator account in the Welcome Screen, you cannot login as Administrator in Normal mode. Boot into Safe Mode to login as Administrator. Please see the following Microsoft Knowledgebase article:
Error Message: Unable to Log You on Because of an Account Restriction:
http://support.microsoft.com/?kbid=290109