Why is adobe flash player plugin installer marked as "important security update"? Is security really compromised if one does not install that?
This happens in Windows all the time. Every couple of months, Windows releases a patch for broken software. What was wrong? Well, now everyone knows, because they had to declare what was broken.
I would take the security update, because the people that don't are now wide open.
Now, if you don't even have flash player on your system, something else is up. Make sure that it is software from Adobe, but either way install it.
Security updates are just that... for security. There should be no risk in taking one from a legitimate source.