Bugbear-B, which morphs its contents and can turn off some firewalls and anti-virus software, accounted for 12% of the reports the company received.
Graham Cluley, senior technology consultant at Sophos, said all 10 viruses were executable Windows 32 viruses spread via e-mail. "Companies can protect themselves by blocking executable code at the e-mail gateway. These viruses should not have had anything like the impact they did," he said.
Eight of the viruses can spread using more than one method such as e-mail, internet relay chat and network sharing - a trend Sophos expects to continue.
The remaining four places in the top five were taken by Klez-H, and variants of the Sobig virus, including Sobig-C, which posed as a support e-mail from Microsoft's Bill Gates. The impact of Sobig makes it the biggest virus threat encountered this year so far.