From: http://www.mvps.org/marksxp/WindowsXP/welskip.php

Skip the Welcome Screen and Logon Automatically

If you wish for your system to automatically logon to a user instead of having to enter a password or select which user you would like the following information should help. This will also be able to restore your automatic logon if you lost it during the installation of the .Net Framework on your machine (ASP.NET Machine Account).

Firstly start by opening your run box and enter control userpasswords2

When the user accounts window opens 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.

If at a later stage you want a password back on your system, follow the same steps but recheck the box. In order to open user accounts you must be using an Administrator level account