You wish that you computer auto login with a special user when you switch it on / reboot it. The solution to do that is quite simple.
1.) start CMD and type:
and press enter.
2.) Select the user account from the list (the account to which you want to automatically logon). Uncheck the option "Users must enter a user name and password to use this computer" and click Ok.
3.) Type the user account password and complete the process.
For Windows XP Home, don't try to auto-login as the built-in Administrator, as you'll receive an error message.