If your Windows XP Professional computer is NOT a member of a domain, you might be using the friendly logon interface.
When a user account is added to the local Administrators group, the built-in Administrator account is no longer available for logon during the friendly logon.
To workaround this issue, use any of the following:
- Press CTRL+ALT+DELETE twice during the friendly logon interface to use the classic logon interface.
- Logon in Safe mode.
- Logon as a user that is a member of the Administrators group.
- Logon remotely using Remote Desktop and the Autologon feature of the Remote Desktop Client.