The MacDefender malware, according to Cult Of Mac, now has a new variant labeled MacGuard. This new variant no longer needs you administrator password to run. You can read more here:
http://www.cultofmac.com/mac-defender-malware-mutates-it-can-now-infect-your-machine-without-administrator-password/97109.

Apple is coming out with a patch soon for Snow Leopard to fix this issue. In the mean time, don’t click on any strange links, turn off “Automatically open safe files” in Safari and other browsers, and delete anything you don’t remember downloading.