Actually, you don't have to buy a Mac. Depending on what kind of hardware you have in a Windows computer you can run Mac OS X on a Windows computer on a second hard drive. See EFI-X technology in the June edition of MaximumPC.
But aside from that there is so much stuff to go over. Watch out for strange emails and don't click on strange websites. A bank is not going to send you an email asking you to confirm your Social Security Number, mother's maiden name, credit card number, etc. Be sure when you do business online when using your credit card that it is a legit business and that you have a secure connection. Common sense like that. Trust no one.
All of the above applies to Mac owners also. Just because somebody owns a Mac does not mean that they will not fall for some of the tricks. And the tricks may come by telephone call rather than over a computer.
Common sense is the first defense against fraud rather than some software program. A software program cannot think for you.
There is not a lot of malware for Macs but it has been repeatedly demonstrated that Macs can be targeted by various exploits. But aside from that anybody could potentially be taken in by a crafty email or a crafty telephone call. Remember the X-Files-'Trust No One.'
Just a thought for you-if you use your credit card to pay for a meal in a restaurant, what is going to stop somebody getting your credit card number right there? Every think about that? How about if somebody was dishonest at a motel or hotel? Check your credit card purchases when you get your bill to pay for stuff you bought using your credit card. You don't have to be using your Windows, Mac, or Linux computer to have your credit card compromised!
Whenever somebody says get a Mac or get Linux to prevent credit card theft, that is just silly. You don't have to be sitting in front of a Windows computer to have your credit card number stolen. There are some criminals that are smart but most are not and a computer is likely going to be one of the least likely ways you get a credit card stolen. Criminals can go through trash, get your credit card number at a restaurant, by telephone, etc. Do you really think that if everybody used Mac computers there would be no credit card theft?