How Can I Cancel Automatic logon in Windows XP

By default, MSGina.dll checks the state of the SHIFT key when AutoAdminLogon is 1. If the SHIFT key is held down during the boot process, MSGina.dll will ignore the AutoAdminLogon key value and prompt the user for identification and authentication information interactively. You can use this method to temporarily disable automatic logon at startup.

or

Click Start, and then click Run. In the Open box, type control userpasswords2, and then click OK. In the dialog box that appears, enable Users must enter a user name and password to use this computer check box, and then click OK.

or

Click Start, Run and type

rundll32 netplwiz.dll,ClearAutoLogon