JS validation Guru PLEASE !!

Hi Folks
Can I do the following please:
In have a form for entering payments of invoices.
If they pay the whole ammount that is ok BUT if they only pay a part payment i wanna do the following.
On the form i have radion buttons (partpay yes & no) - I have the validation in place where they must select one or the other.
If they select yes i want an alert to show "enter this part payment in paidnotes" - This paid notes is a field also in the form.
This field however could already contain text about a previous payment.
What i need to do is send the focus of the "yes" radiobutton to the "paidnotes" field then check that they actually enter some text into that field - whether or not text already exists or not.

As far as i can make out this would be an "onchange" event but i do not have the experience to understand just how to do it.

Sorta like (in Plain english)
if yes is checked - make sure they type sumpin into the textbox "paidnotes" whether it already contains text or not.

The js validation i already have in place is (onsubmit)
if(!document.form1.partpay[0].checked && !document.form1.partpay[1].checked){
alert("Is this A Part Payment or Not?")
return false
}
if(document.form1.partpay[0].checked){
alert("DON'T FORGET To Record this Part Payment into the 'Part Payment Notes' Field");
document.form1.paidnotes.focus();
return false
}


PS "paidnotes" is a textbox field

Can anyone please help me with this.
cheers and thanks in advance
Grabit

You may get a better response by asking this in the JavaScript forum, since that is dedicated to this sort of topic. Also, there are a number of existing validation libraries available (many as plugins to things like jQuery) that will probably do much more than anything you try to build up yourself.