1) Linux is more secure at the kernel level. It's harder to get elevated priveledges on Linux than Windows. Linux isn't a magical pixie land free of viruses and bad stuff. You *can* get viruses on Linux, but they're rarer and can't do damage.
2) No OS is going to protect against stupidity and sneakiness. You can get malware from an ad on a site on Windows - far harder to do so in Linux.
3) Linux is free and doesn't force you to do stupid crap like have a Windows live account just to use your own computer.