The good news revealed by the survey was that the number of firms caught out by viruses had dropped by almost one-third since the last time the study was done in 2004.
This reduced infection rate was put down to the fact that most businesses now use anti-virus software to clean up e-mail traffic before it reaches in-boxes.
Chris Potter, a partner at PricewaterhouseCoopers and one of the report's authors, said firms had got much better at keeping their anti-virus software up to date and patching vulnerable systems.
But, he said, it was hard won experience. "They know how much this can hurt if they get caught out."