function verifica(nome_form){

form = document.getElementById(nome_form);

if(form.busca.value==''){
alert('Digite uma palavra!');
form.busca.focus();
return false
}
return true

}

function abre_pop(url){

jan = centraliza_pop(url,'newsletter','300','200','scrollbars=no, status=no'); //opções do popup

}
function centraliza_pop(theURL,winName,width,height,features) { //Esta função aqui é para o pop-up abrir no centro, está funcionando legal
    var window_width = width;
    var window_height = height;
    var newfeatures= features;
    var window_top = (screen.height-window_height)/2;
    var window_left = (screen.width-window_width)/2;
    newWindow=window.open(''+ theURL + '',''+ winName + '','width=' + window_width + ',height=' + window_height + ',top=' + window_top + ',left=' + window_left + ',' + newfeatures + '');
    newWindow.focus();
}