<style>
<!--
input.ri { visibility: hidden }
-->
</style>

<script>
function Verifica() {

var which = document.nomeform
var pass=false
var html_code = ""
if (document.images){
for (i=0;i<which.length;i++){
var tempobj=which.elements[i]
if (tempobj.type=="checkbox"&&tempobj.checked==true){
pass=true
}
}
}
if (pass==true){
which.B1.style.visibility = "visible";
} else {
which.B1.style.visibility = "hidden";
}
}

</script>

<form method="POST" action="smsdel.asp" name="nomeform">
<input type="checkbox" id=checkbox1 name=del1 onclick="Verifica();" value="10">

<input type="checkbox" id=checkbox2 name=del2 onclick="Verifica();" value="11">

<input type="checkbox" id=checkbox3 name=del3 onclick="Verifica();" value="12">

<input type="checkbox" id=checkbox4 name=del4 onclick="Verifica();" value="13">

<input class="ri" type="submit" value="Invia" name="B1"><input type="reset" value="Reimposta" name="B2">
</form>


Senza mettere un allert faccio il controllo si tutti i checkbox del tuo form e rendo visibile o nascondo il tasto submit