The amount of spam containing dangerous attachments designed to infect PCs with malicious viruses increased by eight times between July and September this year, according to Sophos.
The security firm said one in every 416 email messages received between July and September was spam compared to just one in every 3,333 emails between April and June.
Much of the increase has been caused by several large-scale malware attacks. Sophos said the worst single attack was the Agent-HNY Trojan horse, which was disguised as the Penguin Panic arcade game for Apple iPhones. However, only Windows users are affected by the viruses.
"For Apple Mac and Unix lovers, these major spam attacks just mean a clogged-up inbox, not an infected operating system. But organised criminals are causing havoc for Windows users in the hunt for cold hard cash," said Graham Cluley, senior technology consultant at Sophos.
"Too many people are clicking without thinking - exposing themselves to hackers who are hell-bent on gaining access to confidential information and raiding bank accounts."
The report also identified the US as the nation responsible for most of Q3's unsolicited email, contributing 18.9 percent of the total spam sent. The US was closely followed by Russia, which was responsible for 8.3 percent of spam, and Turkey, which accounted for 8.2 percent.
"The message needs to be heard loud and clear: if you don't properly defend your PC you are not only putting your data, finances, and identity at risk, you are also endangering other members of the internet."