How to login as Administrator in Windows XP?

The built-in Administrator account is hidden from 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 at the Welcome Screen and input your Administrator password in the classic logon window that appears.

To have the Administrator account displayed in the Welcome Screen:

Click Start, Run and type Regedit.exe
Navigate to the following key:

HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \ Winlogon \ SpecialAccounts \ UserList

Use the File, Export option to backup the key
Create a new DWORD Value named Administrator
Double-click Administrator, and set 1 as its data
Exit the Registry Editor.