Event.observe(window, 'load', init, false);

function init(){
     $('username-submit').style.display = 'none';
     Event.observe('username', 'keyup', checkuser, false);
}

function checkuser(){
     var url = '/includes/checkuser.asp';
     var pars = 'username='+escape($F('username'));
     var target = 'username-available';
     var myAjax = new Ajax.Updater(target, url, {method: 'get', parameters: pars});
}