Javascript alert box doesn't disappear in IE everytime I hit submit button.

Hi,
This is my very first day in this website forum. I've been forced to singup with this forum because I am going through a very little but crucial error. Hope I'll get some help.

I've made a 'quotation request' form in my site. It works perfectly in firefox but when I try it in Internet Explorer the alert box keeps coming everytime I hit submit button.

The website is about 16 seater minibus. and the form is about customers requesting quotations for their journey. I am going to copy and paste my codes so you will have a better idea.

JavaScript

var submitcount=0;

function reset() {
document.callback.departure.value="";
document.callback.destination.value="";
document.callback.date1.value="";
document.callback.selection.value="";
document.callback.selection1.value="";
document.callback.passengers.value="";
document.callback.selection2.value="";
document.callback.selection3.value="";
document.callback.name.value="";
document.callback.phone.value="";
document.callback.email.value="";
document.callback.how.value=""
}

function checkFields() {
if (
(document.callback.departure.value=="") ||
(document.callback.destination.value=="") ||
(document.callback.date1.value=="") ||
(document.callback.selection.value=="") ||
(document.callback.selection1.value=="") ||
(document.callback.passengers.value=="") ||
(document.callback.selection2.value=="") ||
(document.callback.selection3.value=="") ||
(document.callback.name.value=="") ||
(document.callback.phone.value=="") ||
(document.callback.email.value=="") ||
(document.callback.how.value==""))
{
alert("Please enter all required fields.");
return false;
}

else
{
if (submitcount == 0)
{
submitcount++;
return true;
}
else
{
alert("This form has already been submitted.");
return false;
}
}
}
// End SCRIPT

Another Javascript
(This javascript is to hide and display Returns date/time)
function display(obj,id1) {
txt = obj.options[obj.selectedIndex].value;
document.getElementById(id1).style.display = 'none';
if ( txt.match(id1) ) {
document.getElementById(id1).style.display = 'table-row';
}
}
End script

So the thing is, it works absolutely fine in firefox but when I open it in IE and fill required fields or even all fields and i recieve an alert message that I've written in JS (Please enter all required fields.). It doesn't go through.

My question is, how can I make it work in IE as well. I have IE8 installed in my PC.
PLEASE HELP!