This technique means that the culprits do not have to be in the same country as the innocent computers they are using to send their spam.
Whilst the United States, South Korea and China still account for over 50% of all spam, the USA and Canada have done well to reduce their contribution to the problem.
Sophos has seen a sharp drop in spam sent from North American computers due to a number of factors, including jail sentences for spammers, tighter legislation and better system security.
"Efforts such as ISPs sharing knowledge on how to crack down on spammers, and authorities enforcing the CAN-SPAM legislation, have helped North America tackle the spammers based on their doorsteps.
Some of the most prolific spammers have been forced to either quit the business or relocate overseas as a result," said Graham Cluley, senior technology consultant for Sophos.
"The introduction of Windows XP SP2 a year ago, with its improved security, has also helped better defend home users from computer hijacking.
Feeling the impact of international awareness and country-specific legislation, spammers are increasingly turning to illegitimate providers to fuel their success and their key partners in crime are virus writers and hackers.
By taking control of unprotected PCs, hackers can relay spam, launch denial-of-service attacks or steal user information, without the computer owners being any-the-wiser.
"There are fortunes to be made from the dark side of the internet, and spammers who are finding it harder to sell goods via bulk email are likely to turn to other criminal activities," continued Cluley.
"What the chart reveals is that spammers and virus writers can exploit unprotected computers anywhere in the world to send out their unwanted messages - everyone has a part to play in the fight against spam."