By default windows XP doesn't show the Administrator in the user list at the welcome screen. Here's a way to get around it.
Now head up to HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\CurrentVersion\Winlogon\SpecialAccounts\Userlist
create a new DWORD entry and name it as Administrator and change its value to 1.
exit and reboot for the changes to take effect.
To change it back change its value to 0 or simply delete the key.