con esto te puedes dar una idea
<script language="Javascript">
function valida(f)
{
if(!f.as[0].checked&&!f.as[1].checked&&!f.as[2].checked)
{
alert("debes de seleccionar al menos un Check Box");
return false
}
else return true
}
</script>
</script>
<center>
<form onsubmit="return valida(this)">
<input type="checkbox" name="as" value="check me">
<input type="checkbox" name="as" value="check me">
<input type="checkbox" name="as" value="check me">
<input type="submit">
</form>
</center>
<center><hr size="1" width="100%" noshade><a href=javascript
:MsgrApp.LaunchIMUI('gpa36@hotmail. com')>
<strong><font face="courier new" color="#666666" size="2">Contact Me?[/CODE]</strong>
<IMG SRC="http://woertman.org:8080/msn/gpa36" border="0"></A></ce