Spam volumes have increased by 140 per cent since March, according to new figures from security giant McAfee, which has recorded its longest run of increasing monthly spam levels.

The McAfee Q2 Threats Report (PDF), released today, said that the rise had been driven by surging growth in botnet activity. Some 14 million new computers were recruited this quarter, an increase of 16 per cent over the previous quarter, representing an average of more than 150,000 computers infected every day.

McAfee also reported growth in password-stealing Trojans as the monetisation of illegal activity on the internet becomes the primary aim.

The cumulative number of such Trojans is around 600,000, nearly double the total which had been discovered by 2008, said McAfee security analyst Greg Day.

The company also reported a surge in so-called Auto-Run malware, which exploits Windows' Auto-Run capabilities and is most commonly spread via USB and portable devices.

Day pointed out that this is something of a throw-back to the days when PCs were infected by floppy disks containing viruses, and warned users to turn off Auto-run.

"The technology has been around for a while but it has slipped a bit under the radar," he said. "It requires simple user education: if you put some [removable storage device] into your PC you should have to browse for the device and find out how to run it. If we can teach them how to use email it can't be that hard."

McAfee also warned social network users to beware of spam and malware spread through compromised accounts and using shortened URLs as bait.