Hi Barry, is this an Intel iMac, or a PPC iMac?
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...