Malware/Virus Flashplayer

Official Flash Player installer, downloaded from adobe.com, does not modify any user settings, such as modifying user's search engine preference.

If you believe this happened after installing Flash Player, you were most likely tricked into downloading a malicious Flash Player installer.  Unfortunatley, there are many malicious actors who post fake/malicious Flash Player update notifications to trick people into downloading/running these fake installers which often include malware/viruses/etc and can modify user settings.

I would recommend uninstalling the Flash Player you downloaded last week, running a complete system scan for viruses, malware, adware, etc, and then download the official Flash Player installer from Adobe and https://get.adobe.com/flashplayer.  When installing Flash Player, select the option 'Allow Adobe to install updates (recommended)'.  With this option selected, Flash Player pings Adobe's servers once per day to check if an update is available.  If an update is available, it automatically downloads and installs Flash Player (and only Flash Player) silently in the background with no user interaction required.  This is similar to how browsers such as Chrome and Firefox download and install updates silently without user interaction.

You don't mention which browser you use, but if you use Chrome, there is no need to install Flash Player separately, as Google embeds Flash Player in Chrome and updates for the embedded version are released by Google.