basic javascript syntax question regarding attributes and functions [SOLVED]

[HoraShadow]

I'm just starting with javascript. I tinkered this very simple function, that enables / disables two textboxes when a checkbox is clicked.

<script language="javascript">
<!--
function enableField(tb1, tb2)
{
    with (tb1)
    {
        disabled = !document.form1.checkbox.checked
    }
    with (tb2)
    {
        disabled = !document.form1.checkbox.checked
    }
}
//-->
</script>

I want to know how can I access the properties of an html object without using the with (attribute) syntax. Basically doing it inline, replacing the name of the object with the attribute I passed to the function.

Instead of:
with (tb2)
{
disabled = !document.form1.checkbox.checked
}

Do:
document.form1.[attribute I passed].disabled = !document.form1.checkbox.checked

How can I construct that dynamic path?

Thank you
HoraShadow