Код:
<script language="JavaScript">
function check(theform) {
if (theform.name1.value=="" || theform.name2.value=="") {
alert("Заполните все поля");
return false;
}
if (theform.name2.value!=theform.name1.value) {
alert ("Поля должны быть равны!");
return false;
}
}
</script>
<form name="form1" action="script.php" method="post" onSubmit="return check(this);">
<input name="name1" type="text">
<input name="name2" type="text">
<input type="submit" value="Send">
</form>