Is there a way we can scan third party software before / after installing into a user machine using Qualys ?
what sort of software? Popular titles in global use are covered and yes you can discover differences before and after installation.
What is your goal? looking for vulnerabilities or just a change in the installed software detected?
You can do a light scan to get the software with authentication or even better the Agent.
Just would like information in your goal.
Retrieving data ...