Flashback - Detect and remove the uprising Mac OS X Trojan...
In order to avoid detection, the installer will first look for the presence of some antivirus tools and other utilities that might be present on a power user's system, which according to F-Secure include the following:
If these tools are found, then the malware deletes itself in an attempt to prevent detection by those who have the means and capability to do so. Many malware programs use this behavior, as was seen in others such as the Tsunami malware bot.
The most current flashback removal instructions are F-Secure's Trojan-Downloader:OSX/Flashback.K.
More bad news...
Removal for 10.5...
How can I get rid of the flashback virus?
1. Flashback is Trojan / Backdoor malware.
2. The symptoms you describe are not those of Flashback, which simply redirects your browser to an ad sites and earlier versions were known to frequently crash applications.
3. Flashback has not been active for several months now.
4. You don't say what type of Mac you have, but if it's PPC then Flashback won't run on it. If it's an Intel Mac you are running an obsolete OS X and should upgrade it as high as you can as quickly as you can.