<script>
function valida(p)
{
if (p.value.substring(0,7) != "http://") p.value="http://"+p.value
raros= ["ñ","á","é","í","ó","ú"]
for (var i=0; i<raros.length; i++)
{
if (p.value.indexOf(raros[i])!=-1)
{
alert('carácter '+raros[i]+' no válido');
}
}
}
</script>
<input type="text" name="pepito" onblur="valida(this)">