Event.observe(window, "load", window_onLoad);

function window_onLoad(event) {
    attachEvents();
}

function attachEvents() {
    Event.observe($("grpSearchCriteria").select("input[type='text']").first(), "keypress", txtSearchCriteria_onKeyPress);
}

function txtSearchCriteria_onKeyPress(event) {
    var iKeyCode = event.which ? event.which : event.keyCode;
    if (iKeyCode == 13) {
        Event.stop(event);
        var oElement = Event.element(event);
        if ((oElement == $("grpSearchCriteria").select("input[type='text']").first()) && oElement.value.length > 0) {
            $("grpSearchCriteria").select("input[type='submit']").first().click();
        }
    }
}