Detect flash plugin, if no plugin --redirect to non-flash site (javascript)

Wanting to know how to redirect a user to another site (non-flash) if they do not have the flash plugin installed on their computer.

–all of this using javascript on the main login-page

if no flash installed on computer, redirect to—non_flash.htm
if flash is detected then redirect to–flash.htm

reason being, some of the people that are using my web apps are old and don;t feel as if they should have to download something on their computer to run my software.
stupid, i know—

Thanks in advance