Many on here will advise against installing any anti-virus apps for Mac as there are, as yet, no viruses that affect Macs (although there is some malware). That said, the one that gets the most recommendations for being trouble-free is the free/donationware clamXav: www.clamxav.com
Whatevery you do, don't install MacKeeper - from reports on here and elsewhere it'll likely render your system useless.
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...