The truth is the Mac is just as vulnerable, but there are far fewer viruses out for them. Leo's general rule of thumb is that Macs tend to be better home computers while Windows is better for businesses. The other thing Windows is really good for is gaming, although the Mac is catching up as a gaming platform. Unless there's specific applications that Gary may need, Leo recommends that he go with a Mac. Because the Mac is less of a target to hackers, he won't have to worry about security as much.
Gary also wants to know what anti-virus and anti-spyware to get. Leo suggests ESET's CyberSecurity for the Mac, which is available at the Apple store (Disclaimer: ESET is a sponsor). There's also a shareware program that could be particularly useful called LittleSnitch. It watches for outbound connections because when someone gets something bad on your system, they want to connect to the outside world. It can be annoying though because it constantly asks whenever something wants to connect to the internet. Leo recommends a Macbook Air if he can afford it, since they are so light and portable.