Microsoft JScript runtime error: '(function name)' undefined

Microsoft JScript runtime error: 'txtGivenName_OnFocus' undefined

After adding what I thought was unrelated javascript code to a web page, I suddenly get errors that suggest the browser cannot find the javascript function, which seems as simple to me as a day in development mode.

I think this is a load sequence order problem. Initially, my script was at the bottom of the page. I did this with the intention of helping the site's SEO rankings.

When I moved the function to the top of the webpage, the error went away. Now he's back.

I have a feeling that someone would suggest a jQuery solution to execute some code only when the page is fully loaded. I don't know jQuery. IfjQuery is listed in the answer, please explain what I need to do (links, script file placement) for RTM VS 2010.

I am trying to set focus to the first textbox on a webpage and pre-select all text in the textbox

Additional Information:

If I disable this validator, the problem goes away:

<asp:CustomValidator ID="valSpecifyOccupation" runat="server" ErrorMessage="Required"
                                            ClientValidationFunction="txtSpecifyOccupation_ClientValidate" 
                                            Display="Dynamic" Enabled="False"></asp:CustomValidator>



function txtSpecifyOccupation_ClientValidate(source, args) {

    var optOccupationRetired = document.getElementById("<%=optOccupationRetired.ClientID %>");

    if (optOccupationRetired.checked) {
        args.IsValid = true;
    }
    else {
        var txtSpecifyOccupation = document.getElementById("<%=txtSpecifyOccupation.ClientID %>");
        args.IsValid = ValidatorTrim(txtSpecifyOccupation.value) != "";
    }

}