dosable a check box

[scriptnewbie26]

i am trying to disable a checkbox 1 when a particular drop down list 5th in the list is selected
can u please help me with that

<div id="Div3">
            <label  class="pLabel">
                Which of the following products have you purchased in the last 2 months. Please
                check all that apply.<br/>
            </label>
            <span >
            <input  type="checkbox" name="CHKBOX_1" value="Product 1" id="Product1" tabindex="5" /> 
               
            <label >
                Product 1</label>
            <input  type="checkbox" name="CHKBOX_2" value="Product 2" id="Product2" tabindex="6"  />
                
            <label >
                Product 2</label>
            <input  type="checkbox" name="CHKBOX_3"  value="Product 3" id="Product3"tabindex="7" />
                
            <label >
                Product 3</label>
                </span>
				<br/><br/>
        </div>

<select name="age">
                <option value="age">Please select your age</option>
                <option value="18-24">18-24 </option>
                <option value="25-34">25-34 </option>
                <option value="35-50">35-50 </option>
				<option value ="50+" > 50+ </option>
                
            </select>

function validation(){
    if ( document.form77.age.selectedIndex == 0 )    {        
        alert ( "Please select your age." );        
            return false;    
            }
            if (document.form77.CHKBOX_1.checked == false && 
                    document.form77.CHKBOX_2.checked == false && 
                         document.form77.CHKBOX_3.checked == false){
                             alert ('Please select a product 1 , 2 or 3');
 return false;
 }
 if (document.form77.age.selected == 4 ){
       document.form77.CHKBOX_1.checked.disabled == true;
       return false;
 }

[007Julien]

See http://www.w3schools.com/jsref/prop_...ox_checked.asp and write :

if (document.form77.age.selected == 4 ){
       document.form77.CHKBOX_1.checked = true;
       return false;}