It appears that many users who have been caught out with Flashback trojan are running Mac OS X 10.5 (Leopard) or older. Unfortunately Apple will not be providing any updates to patch the vulnerability which Flashback exploits.
Therefore you should run the Flashback Checker (and Flashback Removal Tool if needed) mentioned in my previous post and then disable Java in your web browser.
How to disable the Java web plug-in in Safari
http://support.apple.com/kb/HT5241
How to disable the Java web plug-in in Chrome
Visit the Plug-ins page at chrome://plugins/
Find the Java plug-in and click the disable hyperlink
How to disable the Java web plug-in in Firefox
http://support.mozilla.org/en-US/kb/How%20to%20turn%20off%20Java%20applets