If you start getting pop-ups on a light blue background from a company called Security Shield - adivisng you of all sorts of nasties on your hard-drive - you have picked up a seriously nasty virus.
My work PC (I work for BT) has got it - and if it can get through our firewalls and virus protection, it;s got to be good (or is that bad!).
Our tekkies advise that the only way to lose it is for my PC to be re-builkt from a wiped hard drive - so please be very careful when surfing.