[Old Pedant]

First of all, why do you keep repeating document.forms[0] all over the place?

function confirmsubmit()
{
    var f = document.forms[0];
    if ( f.visitor_name.value == "" || f.visitor_name.value == "Enter your name")
    {
        window.alert("You did not enter your name");
        return false;
    }
    ...
    ... similarly throughout ...
    ...
    // last step:
    for ( var cb = 1; cb <= 5; ++cb )
    {
        if ( f["interest"+cb].checked ) return true;
    }
    alert("You must check at least one interest";
    return false;
}