Sys.Application.add_init(login_PageInit);

function login_PageInit(sender, args) {
    var txtUsr = $get('txtUsername');
    if (txtUsr.value != "") {
        var txtPwd = $get('txtPassword');
        txtPwd.focus();
    } else {
        txtUsr.focus();
    }
}

document.onkeypress = function(evt) {
    var keyCode = 0;
    if (document.all) {
        keyCode = event.keyCode;
    }
    else if (document.getElementById) {
        keyCode = evt.which;
    }
    else if (document.layers) {
        keyCode = evt.which;
    }
    if (keyCode == 13) {
        __doPostBack('__Page', 'keypress');
    }
}