I remain surprised at the comfort some people place in their anti-vurs defences. At any given time there will be a tiny percentage of viruses out there to which no anti-virus programme has an answer. Tiny, but it is that percentage that does most damage, by and large, and sooner or later even a thoroughly vaccinated PC is likely to get infected.

The best defence is to have non-Microsoft operating system and non-MS browser both available on another drive. (Linux and Opera are probably best, limited only by the fact that fewer applications are written for them.) You can get effective versions up and running for vertually nothing, and so far they have been much neglected by the virus writers, who are focusing all their energies on Microsoft.