It's not really a virus - more malware. Something that's designed to trick you into getting your credit card information.

Any computer system is only as secure as the person using it. If you're willing to put your admin password in (required before you can install/run something like this) then there's not much that can be done.

Bottom line: Don't ever install programs from a company or website you don't trust.

Second bottom line: BACKUP!

Mac Security Fake AntiVirus