added new inno setup config

DnsServerWindowsSetup/helper.iss

@@ -0,0 +1,40 @@
+[Code]
+{
+    Helper functions
+}
+
+{
+    Checks to see if the installer is an 'upgrade'
+}
+function IsUpgrade: Boolean;
+var
+    Value: string;
+    UninstallKey: string;
+begin
+    UninstallKey := 'Software\Microsoft\Windows\CurrentVersion\Uninstall\' +
+        ExpandConstant('{#SetupSetting("AppId")}') + '_is1';
+    Result := (RegQueryStringValue(HKLM, UninstallKey, 'UninstallString', Value) or
+        RegQueryStringValue(HKCU, UninstallKey, 'UninstallString', Value)) and (Value <> '');
+end;
+
+{
+    Kills a running program by its filename
+}
+procedure TaskKill(fileName: String);
+var
+    ResultCode: Integer;
+begin
+    Exec(ExpandConstant('taskkill.exe'), '/f /im ' + '"' + fileName + '"', '', SW_HIDE, ewWaitUntilTerminated, ResultCode);
+end;
+
+
+{
+    Executes the MSI Uninstall by GUID functionality
+}
+function MsiExecUnins(appId: String): Integer;
+var 
+  ResultCode: Integer;
+begin
+  ShellExec('', 'msiexec.exe', '/x ' + appId + ' /norestart /qb', '', SW_HIDE, ewWaitUntilTerminated, ResultCode);
+  Result := ResultCode;
+end;