MacDefender Malware May 2, 2011
If you see the same dialog as the OP here.
https://discussions.apple.com/thread/3033524?answerId=15118565022#15118565022
How to remove here. http://www.macrumors.com/2011/05/02/new-macdefender-malware-threat-for-mac-os-x/
"Intego today noted the discovery of new malware known as "MACDefender" targeting Mac OS X users via Safari."
"where users report that the malware is popping up directly in Google image searches."
Carolyn