function generateGuid() {
  var result, i, j;
  result = '';
  for(j=0; j<32; j++) {
    if( j == 8 || j == 12|| j == 16|| j == 20) 
      result = result + '-';
    i = Math.floor(Math.random()*16).toString(16).toUpperCase();
    result = result + i;
  }
  return result;
}

var guid = generateGuid();

document.write("<input type=hidden value="+guid+" name='captcha_guid' />");
document.write("<img src='/scripts/captcha.ashx?guid="+guid+"' align='top'>");