Null, there WAS a version of the Trojan that installed with a bogus Flash update. It popped up on a website and required an Admin user name and password. It did not take you to the Adobe website for updating. Was that what you encountered? If so, it was not this particular Flashback Trojan; it was an earlier version that was pretty innocuous.
Was this install as long ago as August? That was Trojan Bash/QHost.WB... but since then your Mac should have recognized it and not allowed you to download it—unless you overrode the warning.
What OSX version are you running? You can check that by clicking on the Apple in the menu bar and selecting "About This Mac."
To avoid this happening again, go into your browsers' preferences and turn off JAVA. You can safely leave Javascript running... just JAVA needs to be off.
In this case, that's true.
But there are good arguments for turning off all scripting in the browsers, too, and some people are fortunate to be able to do that and still get their work done. E.g. NoScript in Firefox.
The arguments are "good", but not "compelling" enough for me, since I need to allow JavaScript to run for a huge number of the things I do every day. So I try to be "careful"...
10.6.8 and I usually use Firefox.