PC users have been urged to scan their computers before 3 February to avoid falling victim to a destructive virus.
Many file types from Microsoft Office are targeted by Nyxem
On that date the Nyxem virus is set to delete Word, Powerpoint, Excel and Acrobat files on infected machines.
Nyxem is thought to have caught out many people by promising porn to those who open the attachments on e-mail messages carrying the virus.
Anti-virus companies have stopped lots of copies, suggesting it had infected a large number of computers.
The Nyxem-E Windows virus first emerged on 16 January and has been steadily racking up victims ever since. Nyxem-E is also known as the Blackmal, MyWife, Kama Sutra, Grew and CME-24 virus.
Helpfully, the virus reports every fresh infection back to an associated website which displays the total via a counter. Late last week the counter was reporting millions of infections, but detective work by security firm Lurhq found that many of these reports were bogus.
However, Lurhq reported that more than 300,000 machines are known to have fallen victim to Nyxem-E.
SAMPLE SUBJECT LINES
Fw: Funny :)
Re: Sex Video
Miss Lebanon 2006
School girl fantasies gone bad
Like many recent viruses, Nyxem tries to spread by making people open attachments on e-mail messages that are infected with the destructive code.
The subject lines and body text of the various messages Nyxem uses vary, but many falsely claim that pornographic videos and pictures are in the attachments.
On infected machines the virus raids address books to find e-mail addresses to send itself to.
The virus also tries to spread by searching for machines on the same local network as any computer it has compromised.
Unlike many recent viruses Nyxem is set to overwrite 11 different types of file on infected machines on the third of every month. The list of files to be over-written includes the most widely used sorts of formats.
Separately, the virus also tries to disable anti-virus software to stop it updating and can also disable the mouse and keyboard on infected machines.
NYXEM FILE TARGETS
DMP - Oracle files
DOC - Word document
MDB - Microsoft Access
MDE - Microsoft Access/Office
PDF - Adobe Acrobat
PPS - PowerPoint slideshow
PPT - PowerPoint
PSD - Photoshop
RAR - Compressed archive
XLS - Excel spreadsheet
ZIP - Compressed file
Users were being urged to update anti-virus software and to scan their system to ensure they had not been caught out. Many anti-virus firms have also produced tools that help clean up infected systems.
Jason Steer, technical consultant at mail filtering firm Ironport, said Nyxem was a throwback to the types of viruses that used to circulate in the early days of computer networks.
"If you go back 10-15 years ago viruses tended to quite malicious," he said. "They were going to re-format your hard disk, delete files and so on."
Pete Simpson, threat lab manager at security firm Clearswift, said: "It's a bit puzzling because script kiddies have largely left the scene.
"It shows a certain intelligence in its design but what's the motive?" he asked, "Pure vandalism does not ring true these days."
Both Mr Steer and Mr Simpson feared that home users would be hardest hit by Nyxem on 3 February.
Most businesses, they said, now have regularly updated anti-virus systems in place and disinfect e-mail traffic before it reaches users' desktops.
By contrast many home users did not regularly patch Windows, update anti-virus or perform full system scans to ensure their machine stays clean. Users were also encouraged to make regular back-ups of any files they want to preserve.