FlashInstall.log is a log file that tracks and reports the status of the Flash Player installation process. There is no harm in deleting the file as a new file will be created on fresh installation of flash player.
While the FlashInstall.log file is created if it doesn't exist, it's preferred to not delete the file as it contains historical data which is useful when troubleshooting certain installation issues. If the file is deleted this data is lost, thus making troubleshooting certain installation issues more difficult. The FAQ was updated to include the note not to delete the FlashInstall.log file after the video was produced.
Okidoke, thank you. I was wondering as I had followed the KB and video for one computer at the time of posting, and had already installed the latest version of Flash ActiveX (everything successful as far as I can tell).
So in the future, I shall leave the .log file in place.