While you can get viruses by unsafe web browsing practices, file sharing, and even production software media, the likeliest source will be email. It makes sense to have anti-virus installed on your PC, and even better to have your email filtered for viruses before it ends up in your inbox.
Is your anti-virus installed? Is it up to date? How do you know it's up to date?
Doesn't a 100% effective solution sound better?