Recent to Sky broadband? Not a parent? Might want to turn this off.

Login to your My Sky iD and set this to off.

If your a parent, turn it on for added child protection.

http://www.sky.com/products/broadband-talk/sky-broadband-shield/features/

I've also seen TalkTalk. Although I'm unfamiliar with how that works it has a separate option for ISP level virus protection that can be enabled separate to the content filter which is interesting.

Google - Project Shield

Project Shield is an initiative to use Google's infrastructure to protect free expression online. The service currently combines Google's DDoS mitigation technologies and Page Speed Service (PSS), which allow websites to serve their content through Google to be better protected from DDoS attacks.
http://projectshield.withgoogle.com/