I cannot install Adobe Flash Player, because it says that I may have to temporarily disable my AV software. However, Symantec will not let me do that. Please advise.
It is not necessary, nor advisable to disable your antivirus when installing Adobe Flash Player or any other software. You can disregard the advisory on the Flash Player download page - there were a couple of AV products that were giving false positives for Flash Player and the notice is intended to address that possibility Norton is aware of the Adobe file and will not interfere with the installation.
If the need should ever arise, you can disable Auto-Protect from a context menu entry by right-clicking the Norton system tray icon. You should never do this for normal installations, since a software installation is exactly the sort of event that Auto-Protect is there to monitor, to check that the program being installed does not contain malware. Usually you would only disable an antivirus program for a major operating system change, such as the installation of a Microsoft Windows Service Pack, when you do not want to take a chance of the antivirus scan interferring with the wholesale changeout of critical system files.