best virus software to purchase?

None.

Helpful Links Regarding Flashback Trojan and Virus Protection

An excellent link to read is Tom Reed's Mac Malware Guide.

Also, visit The XLab FAQs and read the FAQ on malware.

A link to a great User Tip about the trojan: Flashback Trojan User Tip.

To check for the trojan: Anti Flashback Trojan 2.0.4.

A Google search can reveal a variety of alternatives on how the remove the trojan should your computer get infected. This can get you started. Or the preferred method is to use Apple's protection tool: Flashback Malware Removal Tool 1.0.

Or, open Software Update. If you do not have the Apple protection software installed it will download and install it via Software Update. If no update appears that means you either already have it installed or it isn't needed for your system. The software is only available for Leopard, Snow Leopard, and Lion versions of OS X.

Also see Apple's article About Flashback malware.

For general anti-virus protection I only recommend using ClamXav.

Do not purchase any.

OS X already includes everything it needs to protect itself from viruses. You already purchased it. It's part of the purchase price of your iMac, and I presume you paid enough for it.

Third party AV software is not necessary, most of it is useless and some of it is harmful.