My rav frequently discusses contemporary halachic questions. He quotes a general rule from the Gemarrah (I assume it's from Baba Kama or one of the "Baba's") that a person who causes damages is always responsible, even if accidental (Excuse me if I may not be paraphrasing this rule, correctly.)
With this context, the rav claims that if I email you a file that has a virus, you open it, and the file damages your computer, (e.g. - destroys the hard disk or O.S.), I am responsible. My argument is that the sender is probably NOT responsible because:
1 - All computers, these days, are expected to have virus and security protection, and it is the receivers responsibility to scan all potential email viruses.
2 - There is no way to verify that the sender DIDN'T scan the file before uploading it. If we assume that he did, and there was no virus, a virus could have still been picked up during web trasnfer. It's quite posible...