You'd probably have better luck posting future issues over on the Windows 8 forum, as opposed to this, the Windows 7 forum.
TweakUI basically got off the Windows train at XP and while you can get it to work on later versions of Windows, it wouldn't be the brightest of ideas and it doesn't appear to work on Windows 8 period.
Anyway, the first two hits I got on Google from "Windows 8 automatic login" were the following, both of which should describe how to do this. Just beware that this is a rather large security risk and you should probably both stop and really think this over before rushing to implement it. Also make absolutely sure you enter the password in correctly when setting this up or you're going to have a not so fun time trying to fix it.
Again, take the weekend to really think this over. If you have kids or frequent house guests, this might not be such a great idea. Stop and consider all the implications of any random person being able to come along and only needing to turn the unit on to gain full access to anything contained on it. If you still go ahead with it, at least you'll do so having considered that you might be opening pandora's box.
Regardless of what you decide, make sure to create a second administrator level account for the system. Windows has a nasty habit of corrupting profiles on people from time to time and an emergency admin account can save you a whole lot of time and effort trying to fix things.
I got my wife a computer for Christmas that has Windows 8 64-bit on it and every time we turn it on it asked for her password before it will boot up. We would like it if the computer would boot up without ever having to put in the password. She is the only one using it we have no other accounts set up on it. I know with past versions of Windows I have installed TweakUI and use that to login automatically. That is where the problem is, I cannot find a version of TweakUI that is free that will work on it. I am hoping somebody on here can offer some suggestions on how to fix my problem. Thank you very much in advance for any suggestions you might have.