Computer security companies have issued their reports on the top threats in January, singling out a virus that capitalized on a European storm as posing the greatest risk.

Sophos PLC on Thursday singled out the worm that exploited headlines about the storm as the No. 1 threat in January, saying it accounted for nearly half of all malware, or malicious software, seen that month.

The worm — a computer virus designed to copy itself — was responsible for 46.1 per cent of threats, according to Sophos, outpacing by far the Netsky worm at 16.1 per cent, which the company ranked at No. 2 on its list of Top 10 threats.

The worm, dubbed Dorf by Sophos but known by a variety of names such as Peacomm and Downloader-BAI, started to spread through e-mails on Jan. 18 with the subject line "230 dead as storm batters Europe."

Days later, it switched to using love-related subject lines, according to Finland's F-Secure Corp., which spotted the threat in its Kuala Lumpur, Malaysia, lab.

In a report issued on Thursday, Symantec Corp. listed the threat at No. 4 on its own Top 10 tally, still ahead of Netsky at No. 8 but well behind the top-ranked Sober, which includes its own e-mail engine to spread itself. Sober was first found in 2003.

But in its report, Symantec singled out the storm virus, which it calls Peacomm, as a "hot topic".

The hackers who made the virus are believed to be based in Asia and likely hope to create a so-called "zombie" robot network or "botnet." The network of machines could then be remotely used by malicious individuals to send spam e-mails, spread more viruses or steal information through fraudulent phishing e-mails.