How to remove viruses

Not really needed, but see:

Helpful Links Regarding Flashback Trojan and Virus Protection

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

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.

For the DNS Changer malware see the following:

Will your internet service cut off on July 9?

DCWG | DNS Changer Working Group

How to remove the DNS Changer malware

What viruses?

User tip: Viruses, Trojans, Malware - and other aspects of Internet Security - https://discussions.apple.com/docs/DOC-2435

https://discussions.apple.com/thread/3914052?answerId=18259277022#18259277022 - Questioning the effectiveness of proactive scanning.

Thomas Reed's Macintosh Malware page - http://www.reedcorner.net/guides/macvirus/

Linc Davis 2012 post - https://discussions.apple.com/thread/4110991?answerId=18931766022#18931766022

What version of OS X? You may need something if it's not fully up-to-date 10.6.7 or above.