Therewas an eight-fold increase in the amount of spam containing malware during July and September this year, according to Sophos.

According to the security firm, one in every 416 email messages received between July and September this year was spam compared to just one in every 3,333 emails between April and June this year.

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 spammed out disguised as the Penguin Panic arcade game for Apple iPhones. However, it's only Windows users that 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."

Sophos highlighted a rise in the amount of spam being sent via social networking websites such as Facebook and Twitter, and the security firm expects this rise to continue.

The report also identified the US as the nation responsible for most of quarter three's spam, 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 sent 8.2 percent of th this quarter's spam.

"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."