Adapting this javascript to validate multiple checkboxes

I have been using the following to confirm that the user has checked a box before submitting the form. I need to be able to add an additional checkbox and check it the user has selected at least one. any help gratefully received.

<script language="JavaScript" type="text/JavaScript">
<!--
 
function YY_checkform() { //v4.71
//copyright (c)1998,2002 Yaromat.com
  var a=YY_checkform.arguments,oo=true,v='',s='',err=false,r,o,at,o1,t,i,j,ma,rx,cd,cm,cy,dte,at;
  for (i=1; i<a.length;i=i+4){
    if (a[i+1].charAt(0)=='#'){r=true; a[i+1]=a[i+1].substring(1);}else{r=false}
    o=MM_findObj(a[i].replace(/\[\d+\]/ig,""));
    o1=MM_findObj(a[i+1].replace(/\[\d+\]/ig,""));
    v=o.value;t=a[i+2];
    if (o.type=='text'||o.type=='password'||o.type=='hidden'){
      if (r&&v.length==0){err=true}
      if (v.length>0)
      if (t==1){ //fromto
        ma=a[i+1].split('_');if(isNaN(v)||v<ma[0]/1||v > ma[1]/1){err=true}
      } else if (t==2){
        rx=new RegExp("^[\\w\.=-]+@[\\w\\.-]+\\.[a-zA-Z]{2,4}$");if(!rx.test(v))err=true;
      } else if (t==3){ // date
        ma=a[i+1].split("#");at=v.match(ma[0]);
        if(at){
          cd=(at[ma[1]])?at[ma[1]]:1;cm=at[ma[2]]-1;cy=at[ma[3]];
          dte=new Date(cy,cm,cd);
          if(dte.getFullYear()!=cy||dte.getDate()!=cd||dte.getMonth()!=cm){err=true};
        }else{err=true}
      } else if (t==4){ // time
        ma=a[i+1].split("#");at=v.match(ma[0]);if(!at){err=true}
      } else if (t==5){ // check this 2
            if(o1.length)o1=o1[a[i+1].replace(/(.*\[)|(\].*)/ig,"")];
            if(!o1.checked){err=true}
      } else if (t==6){ // the same
            if(v!=MM_findObj(a[i+1]).value){err=true}
      }
    } else
    if (!o.type&&o.length>0&&o[0].type=='radio'){
          at = a[i].match(/(.*)\[(\d+)\].*/i);
          o2=(o.length>1)?o[at[2]]:o;
      if (t==1&&o2&&o2.checked&&o1&&o1.value.length/1==0){err=true}
      if (t==2){
        oo=false;
        for(j=0;j<o.length;j++){oo=oo||o[j].checked}
        if(!oo){s+='* '+a[i+3]+'\n'}
      }
    } else if (o.type=='checkbox'){
      if((t==1&&o.checked==false)||(t==2&&o.checked&&o1&&o1.value.length/1==0)){err=true}
    } else if (o.type=='select-one'||o.type=='select-multiple'){
      if(t==1&&o.selectedIndex/1==0){err=true}
    }else if (o.type=='textarea'){
      if(v.length<a[i+1]){err=true}
    }
    if (err){s+='* '+a[i+3]+'\n'; err=false}
  }
  if (s!=''){alert('The required information is incomplete or contains errors:\t\t\t\t\t\n\n'+s)}
  document.MM_returnValue = (s=='');
}
//-->
</script>
 
 
 
 
<form action="" method="post" enctype="multipart/form-data" name="form1">
    <input type="checkbox" name="checkbox" value="1">
    <input name="Submit" type="submit" onClick="YY_checkform('form1','checkbox','#q','1','Field \'checkbox\' is not valid.');return document.MM_returnValue" value="Submit">
</form>

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