My new question is, let say they click on this link that is linked to a batch file in their computer c: drive. If they have this application installed it will launch the program ...

If this would be possible it would be even more of a security hole than just being able to "test" if a file exists. - Imagine yourself innocently browsing the web, clicking links - when suddenly you start a program on your own machine - for the sake of arguments i'll call it FORMAT.EXE...

Anyway, there is no reason for the users to NEED the application you are talking about, unless it's a browser plugin - and here are ways of testing for them.

Just put a nice explanation on your page why and how the user should have said application installed, and if they decides they really need it, they will.

-- 
Joost       downtime n. The period during which a system
            is error-free and immune from user input.
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