You're going to have to code against the SharePoint Timer Service or
something of the sort.

Another, less-code-intensive approach might be to schedule  a task in Windows
that opens up a form that triggers your workflow; this is a bit convoluted,
but at least it's all very high-level so you don't have to open up Visual
Studio.

Can you give me idea on "Schedule a task in windows that opnes up a form that
trigger my workflow" or any article on this?

I am also thinking on that I will use Sql Job and send  meil thru sp_sendmail
function, but only limitation here is How can I send mail  to a Sharepoint group
in SQL VB Script, I know I can send mail to one person? ANy idea on this?

Still I am interested in 1st options, on 2nd option I am working, but this will
be my last option.

Maybe this article will help:

sharepointmagazine.net/.../the-dog-ate-my-task-
use-sharepoint-designer-to-email-daily-task-reminders