Adding checkbox for validation

Hi everyone I am a complete newbie with javascript and ive used the following Javascript to validate my form:

[code]
function checkShippingAndPaymentInfo()
{
with (window.document.frmCheckout) {
if (isEmpty (txtShippingFirstName, ‘Enter first name’)) {
return false;
} else if (isEmpty (txtShippingLastName, ‘Enter last name’)) {
return false;
} else if (isEmpty(txtShippingAddress1, ‘Enter shipping address’)) {
return false;
} else if (isEmpty(txtShippingPhone, ‘Enter phone number’)) {
return false;
} else if (isEmpty(txtShippingCity, ‘Enter shipping address city’)) {
return false;
} else if (isEmpty(txtShippingPostalCode, ‘Enter the shipping address postal code’)) {
return false;
} else {
return true;
}
}
}
[/code]

I have now created a check box into my form and I need a validation for this so if the checkbox is empty an alert will apear. Prob a simple prob for you lot so if anyone can help me I will appreciate it. Thank You

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