function valid() { 
	var pass=document.frm.passweb.value;
	var user=document.frm.userweb.value;
	if( user == null || user.length == 0 || /^\s+$/.test(user) ) {
	alert('Por favor, ingrese el nombre de usuario');
		return false;
	}
	else if ( pass == null || pass.length == 0 || /^\s+$/.test(pass) ) {
	alert('Por favor, ingrese la contraseņa');
		return false;
	}
	return true;
}

// constants to define the title of the alert and button text.
var ALERT_TITLE = 'Nuez de Aliste dice:';
var ALERT_BUTTON_TEXT = 'Aceptar';
	
// over-ride the alert method only if this a newer browser.
// Older browser will see standard alerts
if(document.getElementById) {
	window.alert = function(txt) {
		createCustomAlert(txt);
	}
}

function createCustomAlert(txt) {
	// shortcut reference to the document object
	d = document;

	// if the modalContainer object already exists in the DOM, bail out.
	if(d.getElementById('modalContainer')) return;

	// create the modalContainer div as a child of the BODY element
	mObj = d.getElementsByTagName('body')[0].appendChild(d.createElement('div'));
	mObj.id = 'modalContainer';
	 // make sure its as tall as it needs to be to overlay all the content on the page
	mObj.style.height = document.documentElement.scrollHeight + 'px';

	// create the DIV that will be the alert 
	alertObj = mObj.appendChild(d.createElement('div'));
	alertObj.id = 'alertBox';
	// MSIE doesnt treat position:fixed correctly, so this compensates for positioning the alert
	if(d.all && !window.opera) alertObj.style.top = document.documentElement.scrollTop + 'px';
	// center the alert box
	alertObj.style.left = (d.documentElement.scrollWidth - alertObj.offsetWidth)/2 + 'px';

	// create an H1 element as the title bar
	h1 = alertObj.appendChild(d.createElement('h1'));
	h1.appendChild(d.createTextNode(ALERT_TITLE));

	// create a paragraph element to contain the txt argument
	msg = alertObj.appendChild(d.createElement('p'));
	msg.innerHTML = txt;
			
	// create an anchor element to use as the confirmation button.
	btn = alertObj.appendChild(d.createElement('a'));
	btn.id = 'closeBtn';
	btn.appendChild(d.createTextNode(ALERT_BUTTON_TEXT));
	btn.href = '#';
	// set up the onclick event to remove the alert when the anchor is clicked
	btn.onclick = function() { removeCustomAlert();return false; }
			
}

// removes the custom alert from the DOM
function removeCustomAlert() {
	document.getElementsByTagName('body')[0].removeChild(document.getElementById('modalContainer'));
}