I run a PC repair shop and we get virus ridden computers day-in day-out.
In my experience it does not matter what OS, antivirus or concoction of security apps you use, nothing out there offers 100% protection, and there is always something that can get past the latest updates.
The bottom line is browsing habits. If you are frequenting dodgy websites (and that means most adult sites unfortunately), or opening every email attachment under the sun and have teenagers installing every toolbar going, you WILL get a virus.
If you have a decent up-to-date security suite and browse sensibly and intelligently, you will afford yourself a level of protection.
Fellas, do yourselves a favour and stick to the good, clean adult sites like y*up*rn.com and don't click any adverts.
Ladies, ignore the vain "You have been tagged in 25 photos" emails from Faceb**k and verify this by logging in directly instead.
Yes there are tricks like not using administrator accounts, but this is not foolproof and can be a PITA.