"It is much easier to get a remote access Trojan into a company than break down a firewall, " Viveros said.

CNN: New 'Trojan Horse' strain may go mainstream

Viveros said there's a growing number of remote access Trojan programs sent via e-mail that can open the backdoor to a user's PC and gather log-ins and passwords to company intranets.

CNN: New 'Trojan Horse' strain may go mainstream

Last week, a Russian security company reported that as many as 600, 000 Macs, most of them in the United States and Canada, had been infected by Flashback, a Trojan horse virus that infiltrates computers, secretly giving someone else remote access to them.

CNN: Apple: Update will fix Mac 'Flashback' virus