Alert på dette scripts.... hvordan

Hvordan får jeg en alert frem hvis man
glemmer at udfylde felterne med navn ,
adresse, tlf. osv. Alerten skulle være
f.eks. "navn skal udfyldes" og "adresse
skal udfyldes" osv.
<html>
<head>

<script language="JavaScript">
<!--
var resultat;
function beregn() {
    resultat = 0;
    with(document.minForm) {
        for (i=0; i<length; i++) {
            if (elements[i].name.indexOf('antal') != -1 && elements[i+1].name.indexOf('pris') != -1) {
                resultat = eval(resultat+(elements[i].value)*(elements[i+1].value));
            }
        }
        total.value = resultat;
    }


}

// -->
</script>


</head>

<body><font face="tahoma" size="2" color="black">

<form name=minForm>
<table cellspacing="0" cellpadding="0" border="0">
<tr><td colspan="2" valign="top"><font face="tahoma" size="2" color="black">
<input TYPE="hidden" value=75 name=antal1>
<input type="hidden"value=1 name=pris1><br>
Madras Hansen pris: 1000 kr.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<input type="hidden" value=1000 name=antal2> 
<input type=text name=pris2 size="2"> stk.<br>
PH lampe model 3120 pris: 1995 kr.<input type="hidden" value=1995 name=antal3>
<input type=text name=pris3 size="2"> stk.<br>
<input type="hidden" value=5000 name=antal4> 
Stol model Gerda 5000 kr.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<input type=text name=pris4 size="2"> stk.<br>
<input type="hidden" value=100 name=antal5>
Billede af Claude Monet pris 100 kr.&nbsp;&nbsp;<input type=text name=pris5 size="2"> stk.<br>
<input type="hidden" value=200 name=antal6> 
Billede af Claude Monet pris 200 kr.&nbsp;&nbsp;<input type=text name=pris6 size="2"> stk.<p>
Tallet er incl.moms & 75 kr. i fragt <input type=text name=total size="6">&nbsp;<input type=button value=' Beregn ' onclick=beregn();><p>


<b>Udfyld skemaet og tryk p&aring; "Send.."</b><br><br></td></tr>
<tr><td><font face="tahoma" size="2" color="black">Fornavn </td><td><input type="text" name="Fornavn" size="30" onchange=beregn()>
<tr><td><font face="tahoma" size="2" color="black">Efternavn </td><td><input type="text" name="Fornavn" size="30">
<tr><td><font face="tahoma" size="2" color="black">Adresse</td><td><input type="text" name="Adresse" size="30">
<tr><td><font face="tahoma" size="2" color="black">Postnr/by</td><td><input type="text" name="Postnr" size="4">&nbsp;&nbsp;&nbsp;<input type="text" name="By" size="22">
<tr><td><font face="tahoma" size="2" color="black">Evt. email</td><td><input type="text" name="Email" size="20">
<tr><td><font face="tahoma" size="2" color="black">Telefon</td><td><input type="text" name="Telefon" size="20">

<tr><td valign="top">
</td></tr></table><p>
<input type="Reset" value="Slet form">&nbsp;<input type="submit"style="background-color:#1C4680;color:#FFFFFF;" value="Send bestilling">

</form>

</body>
</html>