More than 97% of all e-mails sent over the net are unwanted, according to a Microsoft security report.
The e-mails are dominated by spam adverts for drugs, and general product pitches and often have malicious attachments.
The report found that the global ratio of infected machines was 8.6 for every 1,000 uninfected machines.
It also found that Office document attachments and PDF files were increasingly being targeted by hackers.
