form method="post" id="login_Check" class="form">
       <input type="text" name="naam" class="nameField" value="Gebruikersnaam/e-mail" onfocus="if(this.value == 'Gebruikersnaam/e-mail'){this.value=''};"  
       onblur="if(this.value == ''){this.value='Gebruikersnaam/e-mail'};" />
       <input type="password" name="ww" class="wwField" value="wachtwoord" onfocus="if(this.value == 'wachtwoord'){this.value=''};"  
       onblur="if(this.value == ''){this.value='wachtwoord'};" />
       <input type="checkbox" name="cookie" class="cookieField"/><span>Onthoud mij</span>
       <input type="submit" name="submit" onclick="loginCheck()" value="" class="button" />
      </form>
      <div id="gegevens"></div>

/// LOGIN CHECK ///
function loginCheck()
{
	var xmlHttp = check();
	var gegevens = document.getElementById('login_Check').value;
 
	xmlHttp.onreadystatechange=function()
	{
      if(xmlHttp.readyState==4)
        {
			document.getElementById('gegevens').innerHTML=xmlHttp.responseText;
        }
	}
	  xmlHttp.open("GET","verwerkJs.php?object=loginCheck&gegevens="+gegevens,true);
	  xmlHttp.send(null);
}

<form method="post" action="" onsubmit="functie(); return false;"></form>