If you've created an account in addition to the Administrator account in Windows XP, the administrator account will not be shown in the Logon Scree, this tutorial explains you how to add the Administrator account to the logon screen.
If you are using Windows XP Pro follow these steps,
1. In the Start Menu, select Run.
2. In the Run dialog, type 'regedit' without quotes, to start the registry editor.
3. Navigate to the key,
HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \Winlogon \SpecialAccounts \UserList
4. In the right pane, right click and select, New > Dword
5. A new item will be created, right click on it and rename it to 'Administrator'.
6. Then double click on the Administrator item and enter a value, '1' in the dialog box.
7. Close registry editor and restart your computer, you can see the Administrator account listed in the Login Screen.
To perform the above steps you've to login with a admin user account.
If the new account you've created is a limited user account, then you can access the administrator account, by pressing ctrl+alt+delete keys two times in the login screen. You'll get a login window. enter the username and password. Then do the above steps.
If you are using Windows XP Home Edition, you've to boot into safe mode to perform the above steps. You can enter into safe mode by pressing the F8 key during the bootup process of the computer, and selecting Safe Mode from the list displayed.
Still to make things easier, there is a precoded executable that will do this automatically. This file is no virus or anything harmful. i've personally tested this file. Get the file here,