"Unprecedented" numbers of Windows virus variants are spreading online, say computer security firms.
Infected machines will beep on 2 March
Current top of the threats is Netsky.D, the fourth version of an e-mail worm that scours networks seeking new addresses to send itself to.
Users will know if their PC has caught the virus because infected machines will beep at certain times on 2 March.
Many of the viruses on the list of top threats are variants of others that appeared only in the last few weeks.
The original Netsky virus appeared on 16 February but since then has spawned seven variants.
The most successful of these offspring is Netsky.D that was first seen on 1 March but has spread so fast that many anti-virus companies are already issuing warnings about it.
NETSKY.D SUBJECT LINES
Re: Your details
Re: Your document
Re: Your text
Re: Excel file
Re: Word file
Re: My details
Re: Your music
Re: Your bill
Re: Your letter
Re: Your website
Re: Your product
Re: Your document
Re: Your software
Re: Your archive
Re: Your picture
Re: Here is the document
Netsky.D can infect machines running Windows 95, 98, 2000, ME and XP.
Like many other recent Windows viruses Netsky.D relies on curious users to open the e-mail it travels in and click on the attachment it carries to launch the malicious program.
It disguises itself using a small number of subject lines and names for the attachment. But users can spot the virus because all the fake attachments bear the same .pif suffix.
Once launched the virus scours all the hard drives on networks connected to an infected machine.
"So through infecting just one computer, it can get hold of all e-mail addresses stored on a corporate computer network, maybe some 50,0000 in all, from just a single infected machine," said Mikko Hyppoenen of anti-virus firm F-Secure.
Do not click
Netsky.D avoids sending itself to anti-virus firms and Microsoft in a bid to maximise the time it gets to spread.
The virus also causes infected machines to beep on 2 March between 6:00 and 9:00 AM local time. Each beep will last a random amount of time and be at a random frequency.
Inside the virus is a message mentioning the Czech Skynet website and an organisation called the AntiHacker Crew.
As well as warning about Netsky.D some security firms said the variants of recent viruses were turning up in large numbers.
For instance the Bagle worm was discovered on 18 January and has now spawned eight variants. There are eight different versions of Netsky in circulation and MyDoom has given rise to five new versions as well as some other copycats.
Security firms said users should be wary of messages they were not expecting that arrive bearing attachments.
They added that users should not click on attachments if they do not know what they contain.