<form method="post" action="" >
<p><label for="login">Логин</label><input type="text" name="login" id="login"></p>
<p><label for="pass">Пароль</label><input type="password" name="pass" id="pass"></p>
<p><button id="button" type="submit" disabled>Вход</button></p>
</form>

$("#login").change(function(){
	login =  $("#login").val();
        var expLogin = /^[a-zA-Z0-9_]+$/g;
	var resLogin =  login.search(expLogin);
	if(resLogin ==  -1){
		$("#login").next().hide().text("Неверный  логин").css("color","red").
fadeIn(400);
		$("#login").removeClass().addClass("inputRed");
		loginStat  = 0;
		buttonOnAndOff();
	}else{
loginStat  = 1;
	buttonOnAndOff();
}

function  buttonOnAndOff(){
	if(loginStat  == 1){
		$("#submit").removeAttr("disabled");
	}else{
		$("#submit").attr("disabled","disabled");
	}
}

var form = $('#myForm'),
    login = $('#login'),
    pass = $('#pass'),
    submit = $('#button');
form.change(function(){
    if(login[0].value != '' && pass[0].value != ''){
        button.disabled = false;
    }else{
        button.disabled = true;
    }
})