Step 1. WIN+R (Run) type "CONTROL USERPASSWORDS2" and hit enter.
Step 2. Under the Users tab. Uncheck "Users must enter a user name and password to use this computer."
Step 3. Click Apply. It will have a window pop up "Automatically Log On".
Step 4. Enter the username and password(confirm password also) and hit ok.
And everything is done, next time you boot up it will log in with the specified user account you entered in step 4.
P.S. this isnt a smart thing to do for security reasons.