function CheckAlphabets(e) {

    var whichCode = 0;
    if (window.event)
        whichCode = window.event.keyCode
    else if (e)
        whichCode = e.which;

    if ((c>=65 && c<=90) || (c>=97 && c<=122) || (c==32)) {
        return true;
    }
    else {
        return false;
    }

}

<asp:TextBox ID="txtName" onKeyDown=''javascript: return CheckAlphabets(event);'' runat="server" />