Mac users should be on guard against a fake Mac security tool being distributed online, says the Intego security firm. The program, called MacGuard, claims to scan a computer for "adware, spyware and trojans," and then eliminate them; in reality the app is actually a version of existing Windows malware, which has already infected as many as 30 million people worldwide. The latter assumes control of a person's computer, and displays messages warning about a false infection in an attempt to get users to pay money.