I usually right click a flie (folder, exe, rar) and hit the "scan files with avira" button to scan it for anything that could possible harm my Computer. It then would bring up the Avira Scan window and scan the files and give me a report. Now when I r-click and hit "Scan selected files with Avira.." it doesn't bring up the scan window. I looked in the taskmanager to see if the scanner even launched and it does for like 3 seconds before exiting. I don't see the window yet it says it launched, but it only launches for 3 seconds anyways (not enough time to even scan it)
the free version is the same as the paid mate. as far as the anti virus app goes.
the only difference between the free and paid is that you get multiple updates a day as opposed to 1 for the free version and no popup add.
but yeah uninstalling and reinstalling the av should get it working again.
that being said make sure you scan with another app like alware bytes just to make sure its not something malicious stopping it from running.