I typically use FFmpeg for conversions. It’s a bit more confusing to use than Handbrake, but I find that it’s faster, and the output is generally a higher quality too. Another benefit over Handbrake is that Handbrake will try to re-transcode the entire video; whereas FFmpeg can simply repackage the video and audio tracks into any other format container. In other words, it can convert your file to a compatible file type without losing any video quality.

If you just need to convert it from .avi to, for example, .mp4, you can do that really easily and quickly with this command (remove the square brackets and add the file name):

ffmpeg -i "[source file].avi" -c:v copy -c:a copy "[output file].mp4"

(In layman’s terms, this means “Run FFmpeg. Use [source file].avi as the input file. Copy the video and audio tracks without transcoding them, and output them into [output file].mp4”)

If you need to convert the audio codec - aac audio tends to work well when I’m editing - you could do that with:

ffmpeg -i "[source file].avi" -c:v copy -c:a:1 aac "[output file].mp4"

(Run FFmpeg. Use [source file].avi as the input file. Copy the video track without transcoding it. Convert the 1st audio track to .aac, and output the video and the audio track into [output file].mp4")