Try this -
1. At the command prompt type: rundll32 netplwiz.dll,UsersRunDll
2. In the User Accounts dialog that appears, clear the "Users must enter a user name and password to use this computer" check box and click OK
3. Enter the user name and password of the account you want to automatically want to log on.
Have fun
Please note: This will not work if you are joinrd to a Domain.