var secs = 10;
var wait = secs * 1000;
document.sform.submitbtn.disabled=true;

for(i=1;i<=secs;i++) {
 window.setTimeout("update(" + i + ")", i * 1000);
}

window.setTimeout("timer()", wait);

function update(num) {
 if(num == (wait/1000)) {
  document.sform.submitbtn.value = "Download ";
 }
 else {
  printnr = (wait/1000)-num;
  document.sform.submitbtn.value = "Download in (" + printnr + ")";
 }
}

function timer() {
 document.sform.submitbtn.disabled=false;
}