Si no entendí mal, esto es lo que estás necesitando...
Código:
<html>
<head>
<title> Uno solo </title>
<script language="JavaScript">
<!--
function checkForm(frm){
var x = 0, z = 0;
for (i=0;i<document.forms[0].length;i++){
if ((document.forms[0].elements[ i].type) == "checkbox"){
z = (document.forms[0].elements[ i].checked)?1:0;
x = x + z;
}
}
if (x < 1){
alert("No marcó ninguno");
return false;
}
else{
if(x==1){
document.forms[0].submit();
}
else {
n= confirm("Quiere desmarcar lo seleccionado");
if(n){
desmarcarTodos();
return false;
}
}
}
}
function desmarcarTodos(){
for (i=0;i<document.forms[0].length;i++){
if ((document.forms[0].elements[ i].type) == "checkbox"){
document.forms[0].elements[ i].checked = false;
}
}
}
//-->
</script>
</head>
<body bgcolor="#FFFFFF">
<form method="post" action="" onsubmit="return checkForm(this);">
Marcar:<br>
<input type="checkbox"> <br>
<input type="checkbox"> <br>
<input type="checkbox"> <br>
<input type="checkbox"> <br>
<input type="checkbox"> <br>
<input type="submit">
</form>
</body>
</html>
<hr noshade size=1><img src="http://www.gograph.com/Images-8712/ClipArt/cat03.gif" height="50" border=0 align="absmiddle"> <font size="2" face="verdana" color="#000000">
bet[/CODE]