Auto-Logon

For news on Windows XP:

For manual
Windows XP tweaks:

Auto-Logon

Use this feature of Tweak-XP to force Windows XP to pass over the 'Enter user password' dialog on startup. Just enter your default user details below and check the 'Enable Auto-Logon' checkbox to enable the Windows XP Auto-Logon. Once the Auto-Logon is enabled, Windows XP will log on the user you've specified - there won't be any need to enter your user details on startup.

How to use this feature:

Enter the username you want to log in automatically on startup
Enter the domain name your computer belongs to (if you do not know your domain name, check the 'System Properties' of your Windows XP machine (control panel >>> 'System') - you'll find your domain name there)
Enter the default password for the user you want to logon automatically
Check the 'Enable Windows XP Auto-Logon' checkbox

For some additional auto-logon settings, click the 'Other Auto-Logon settings' button:

Disable the Auto-Logon 'Shift Override' feature: When using the automatic login feature it is possible for a user to hold the Shift key to bypass the login sequence and enter a username and password. This feature disables the ability to override the function.
Limit the Number of Automatic Logins: This option is used to limit the number of automatic logins. Once the limit has been reached the auto logon feature will be disabled and the system will display the standard authentication box.