<form name=form> 
<input type="radio" name="check" value="activ" onClick="activ=1; document.form.inputfeld;"><font class="stn"> aktivieren</font><br> 
<input type="radio" name="check" value="activ" onClick="activ=0; document.form.inputfeld;" checked><font class="stn"> deaktivieren</font><br> 
<input type="text" name="inputfeld" value='javarea.de' onFocus="if (!activ)this.blur();" onChange="if (!activ)this.blur();" size=30> 
</form>

<script language="JavaScript">
function skip () {
this.blur();
}
function toggleField (field) {
if (document.all || document.getElementById)
field.readOnly = !field.readOnly;
else if (field.onfocus)
field.onfocus = null
else field.onfocus = skip;
}
</script>

<form name="formName">
<input type="button" value="enable/disable" ONCLICK="toggleField(document.tn_hinzu);">
<input type="text" name="fieldName" value="jswelt.de">
</form>

document.forms["nameDesFormulars"]

document.forms[0]

// referenz aufs Formular holen
var myForm =  document.forms[0];
// Collection der Elemente holen
var formElements = myForm.elements;

window.alert("Die Anzahl aller Elemente ist " + formElements.length");