So I've gone down the path of Javascript and cookies, but I'm coming up short and I cannot figure out what's wrong.

In the following code, I've been able to successfully create a cookie called "apex__isValid" and set it to true or false based on the state of a checkbox field called "IsValid__c". I use the onClick event to invoke a Javascript function called "setCookie" and it all works like a charm.

The problem is with the "checkCookie" function. My goal is to set the "checked" value of the IsValid__c field based on the current value of the apex_isValid cookie. I have a Javascript function that runs on the onLoad event and invokes the checkCookie function. The line in the Javascript that does not seem to be working is:

      document.getElementById("{!$Component.ValidCheckbox}").checked = isValid;

The IsValid__c field is not affected by the Javascript and I suspect the above line is not working or is not being used in its proper context, but I have no idea what to try next. Please help.

Enclosed is my complete page. I can include the controller extension too if anyone would like to see it. The controller extension is being used to provide a "Save & New" capability for my custom object.

<apex:page standardController="Proxy__c" extensions="CustomExtensionController" sidebar="false">

    <script type="text/javascript" src="/js/functions.js"></script>
    <script src="/soap/ajax/11.1/connection.js"></script>
    <script language="javascript">

//------------------------------------------------------------------
    function getCookie(c_name)
    {
    var i,x,y,ARRcookies=document.cookie.split(";");
    for (i=0;i<ARRcookies.length;i++)
      {
      x=ARRcookies[i].substr(0,ARRcookies[i].indexOf("="));
      y=ARRcookies[i].substr(ARRcookies[i].indexOf("=")+1);
      x=x.replace(/^\s+|\s+$/g,"");
      if (x==c_name)
        {
        return unescape(y);
        }
      }
    }
//------------------------------------------------------------------
    function setCookie(c_name,value,exdays)
    {
    var exdate=new Date();
    exdate.setDate(exdate.getDate() + exdays);
    var c_value=escape(value) + ((exdays==null) ? "" : "; expires="+exdate.toUTCString());
    document.cookie=c_name + "=" + c_value;
    }
//------------------------------------------------------------------
    function checkCookie()
    {
    var isValid=getCookie("apex__isValid");
    if (isValid!=null)
      {
//    alert(isValid);
      document.getElementById("{!$Component.ValidCheckbox}").checked = isValid;
      }
    else
        {
        setCookie("apex__isValid","True",3);
        }
      }
//-------------------------------------------------------------------     
//Function below checks the isValid cookie onLoading of the page
    var previousOnload = window.onload;        
    window.onload = function() {
        if (previousOnload) {
            previousOnload();
        }
    checkCookie()
    }
</script>

  <apex:sectionHeader title="Proxy Edit"
                      subtitle="New Proxy"/>
  <apex:form >
    <apex:pageBlock title="Proxy Edit" id="thePageBlock"
                    mode="edit">
      <apex:pageMessages />
      <apex:pageBlockButtons >
        <apex:commandButton value="Save"  action="{!saveAndNew}"/>
        <apex:commandButton value="Cancel" action="{!cancel}"/>
      </apex:pageBlockButtons>
        <apex:pageBlockSection title="Proxy Information" columns="1" id="thePageBlockSection">
          <apex:inputField value="{!Proxy__c.Policy_Number__c}"/>
          <apex:inputField value="{!Proxy__c.Receipt_Date__c}"/>
          <apex:inputCheckbox value="{!Proxy__c.IsValid__c}" id="ValidCheckbox" onclick="setCookie('apex__isValid',document.getElementById('{!$Component.ValidCheckbox}').checked,3)"/>       
        </apex:pageBlockSection>        
        <apex:pageBlockSection title="System Information" columns="1">
          <apex:inputField value="{!Proxy__c.Scanned_By__c}"/>
          <apex:inputField value="{!Proxy__c.Scan_Date__c}"/>       
        </apex:pageBlockSection>
    </apex:pageBlock>
  </apex:form>
</apex:page>