Greg is worried he's going to be nailed by Cyptowall. If he were to be infected, would he have some warning? Leo says you can sometimes see it happening, but it doesn't give you a warning. It's not instant though, in that it takes time to encrypt the data and if he has a hot backup, always backing up, the encrypted files can infect the backup. Having an offline backup will guard against that.
Greg should check out this white paper from Carbonite. But it's crucial to always keep the OS patched to guard against zero day exploits. He shouldn't click on attachments, and avoid websites that haven't been updated. Unfortunately, no antivirus can guard against it because the attackers can just adjust it every day. The creators of Cryptowall have made over $300 million shaking down users with this virus.