Textbox/check box linking help needed.

Hi,

I'm new to Javascript and am trying to learn as I go but I was hoping someone could help me out with some script and implementation tips for the following scenario:

As part of a form, I have four text boxes and a check box.  If the user enters anything into any one of the text boxes I want the check box to automatically be ticked.  If the user unticks the check box, I want the four textboxes to be cleared.

So far I have got the checkbox ticking automatically by adding the below custom calculation script to the text boxes (name altered for simplicity sake).

if (event.value=="")
{this.getField("textbox1").value="Off"}
else
{this.getField(textbox1").value="Yes"}

This code causes an issue though, in that because there is text in the textbox, I cannot untick the checkbox.

Can anyone offer any help please?  Thanks in advance.

G.