There are a lot of different ways to make a custom Sign up box for wordpress. You could use our Simple Form Generator to get the HTML POST information. However this will only allow new users to sign up, not existing users that already have information in your account.
What I would personally do is create a place on your theme for a widget and place the HTML code in the widget. That way you can enter whatever styling you want and change it on the fly whenever you need to. The alternative is to place it into a theme directly or to put it into a post or page.
There are a few plugins that I know that should work which are made by Katz, which you can find here. The others are searchable throughout the plug-in area of the wordpress admin.