The account must already exist before you do this.
1. Click the Start and type ‘netplwiz’ and hit enter.
2. In the window that comes up, un-check the “Users must enter a user name and password to use this computer’ box, then hit ‘Apply’.
3. In the new dialog box that opens type the name of the account you want to auto-login by default. If the account is not normally passworded then leave the password blank, if the account is passworded then enter the password. Click OK your way out of both windows.