Reader Jon Nunes has old movies and a new operating system that don't see eye to eye. He writes:
I have a lot of old .avi movies from the days when I was using a Windows PC. I'd like to play them on my Mac but I can't seem to when running Mavericks. What should I do?
You're correct that QuickTime Player X is incapable of playing .avi movies, but then it never did natively. Instead you'd have to install something like the Perian QuickTime plugin, which allowed you to play a wide variety of video files, including the many formats gathered under the .avi umbrella. Regrettably the team behind Perian has chosen to halt development. In addition, the foundation underlying QuickTime has changed with OS X Mavericks so that these older plugins no longer work.
You're not entirely out of luck, however. VideoLAN's free VLC Media Player can play these files. Just launch the application and, in the resulting VLC media player window, drag in the movie you want to watch and it will start playing (see image above).
But suppose you'd like to watch your movies outside of VLC? I'd suggest converting them to a format that's compatible with Mavericks--MPEG-4 (H.264) for example. VLC can output a converted file but I've found the results of its work a little iffy--it can drop frames, making for stuttering video.
There are multiple tools available for doing this conversion (some which are simply copies of others) but I wouldn't bother paying for one. Instead, download a copy of the free HandBrake. Among its many talents, HandBrake can convert these movies into a format playable on all Apple devices.
To do the job launch HandBrake and in the navigation sheet that appears, select the movie you want to convert. If the presets pane isn't showing click the Toggle Presets tab. From the list of presets on the right choose the one you'd like to use for your movie (Apple TV 2, for example). Click the Start button and HandBrake will set about converting the movie for you.