Retrieve a list of browsers/agents installed in client system
You want to access the data from the client side so from the conceptual/logically its not good to access the client system. You have to use some medium which run on client side on behalf of server. 
JavaScript and JavaApplet is good in this. You can get the data by JavaScript or Applet and in backend you can send data to the server.
I would like to write a web application (in django) which scans the client/remote computers (assumption is windows) and retrieve the list of software's(mainly browsers) installed. Looking for suggestions to implement it.

Is this possible without asking the user to download any scritps/exe's?
If so, is it possible via java script?

I am planning to use python/django to write the entire app. Any input would be much appreciated.
EDIT : Comments on feasibility in java also much appreciated