New
#1
Change the default application for the file types away from the broken app, remove the app, restart the PC, install the app again and again make it the default.
Anyway, what you should do is a clean and new installation of Windows. What you have now already sounds pretty broken and you don't know, which other symptoms of brokenness you are still to find.
As I said: Change the default application for the file types away from the broken app, remove the app, restart the PC, install the app again and again make it the default. However, I just have to repeat that: Keeping a system, of which you know that it has been infected and of which you cannot tell, if it is really clean, is not recommended. Doing a reinstall is the only right solution.