window.onload=function(){
document.forms[0].onsubmit=function(){
   return checkIt();
  }
 }

function checkIt() {
if(document.getElementById('tos').checked==false) {
   alert('Please read the Terms of Service and check "I have read and accept the Terms of Service" box to continue.');
   document.getElementById('tos').className='highlight'
   return false;
  }
else {
   document.getElementById('tos').className='';
   return true;
  }
 }