The new virus, called Welchia, attacks Lovsan, a version of the Blaster virus which exploits a loophole in Microsoft's Windows operating system, according to F-Secure France.
Like Lovsan, Welchia enters a vulnerable computer that is hooked up to the Internet, F-Secure’s managing director, Alexandre Durante, said.
"It then deactivates the A version of Lovsan, thus disinfecting the machine. It installs Microsoft’s software ‘patch’ to close the loophole exploited by Lovsan and then self-destructs when these tasks have been carried out," he said.
He added, "The virus therefore has an anti-virus action. We have seen anti-virus viruses into the past, but we have never seen one which installs Microsoft patches."
Durante warned though, "the virus is not perfect and may cause other kinds of problems for users."
The US virus fighter Symantec said Welchia presented a problem for corporate intranets as it is continually on the prowl for new computers to penetrate.
This gobbles up bandwidth – the capacity to transmit data – and thus slows messaging, it said.