If you wish your system to automatically logon to a user instead of having to enter a password or select which user. Follow the steps below to Skip the Welcome Screen and Logon Automatically In Windows XP:
In order to open user accounts you must be using an Administrator level account
Click Start--> Click Run--> Type control userpasswords2
Uncheck the box which states 'Users must enter a username and password to use this computer' and apply the setting. You will be requested for the username and password of the account you wish to automatically logon as for following reboots of your system.
At a later stage if you want a password back on your system, follow the same steps but recheck the box.