Hackers and spammers have "raised their onslaught" with two global email-borne virus attacks launched in December and January.

The attacks were so large that they drove up the level of viruses up by a factor of 20 compared with usual activity, according to data from security firm Postini.

The January attack became known as the Storm worm because the original email subject line was '230 dead as storm batters Europe'. At the time of the email, there was a heavy storm occurring in Europe.

The email that contained the virus frequently mutated to show dozens of different sensational but believable headlines designed to tempt the reader into clicking on an attachment and infecting their computer.

Other subjects included 'Russian missile shot down USA aircraft' and 'Saddam Hussein alive!'.

The infectious email had a file attachment containing a Trojan known as 'Downloader-BAI' or 'AUTH-W32/Downloader'.

If a user clicked on the attachment, their computer was infected with the virus which then attempted to send personal information back to the hackers who created the virus.

The virus also provided a back-door for hackers to take control of the computer and add it to a botnet to be used in future spam and virus attacks.

The Storm worm followed on the heels of another email-borne virus, the Happy New Year worm, which attacked email users in late December.

This worm contained a subject line and an attachment exploiting the expectations of legitimate postcards and greetings from friends and families.

Infected attachments contained numerous strains of malicious code (including Tibs, Nuwar, Banwarum, and Glowa) as well as two root-kits designed to hide the presence of the malicious code from antivirus scans.

The two attacks were by far the largest to occur in the past 12 months, according to Postini.

"The explosion of botnets has changed the balance of power in the world of communications security," said Daniel Druker, executive vice president of marketing at Postini.

"As Valentine's Day approaches, email users should continue to keep their guard up, as there are already new mutations of the Storm worm with love-related subject lines."