A malicious program – or 'worm' – has succeeded in attacking more than three million PCs, and experts predict the final number could be far higher.
Variously known as Conficker, Kido or Downup, the virus spreads through memory sticks and low-security networks.
Microsoft says the malicious program works by looking for a file called 'services.exe', and then becomes a part of that code.
It copies itself into the Windows system folder as a file type called "dll". Then It gives itself a 5-8 character name, and modifies the Registry to run the infected dll file as a service.
More after the break
Once the worm is up and running, it creates an HTTP server, resets a machine's System Restore point, then downloads files from the hacker's web site.
Experts recommend PC users should have up-to-date anti-virus software, and should install Microsoft's MS08-067 patch.
You have been warned.