Validation form JavaScript
It mostly looks fine. Some things I would change:

var userName = document.forms["register"]["username"].value;
var passWord = document.forms["register"]["password"].value;


I would extract the repetition here to get the form. This makes the code cleaner and makes it simpler if you change the name of the form.

var form     = document.forms["register"];
var userName = form["username"].value;
var passWord = form["password"].value;


Here 

if (userName == null || userName == "") {


You could just use

if (!userName == null) {


One last thing is that you probably want to check and remove leading and trailing spaces in the user name (and possibly in the password). There is a built in trim method you can use if you're not targeting IE8.