Nachi-B searches for unpatched computers and takes advantage of the same critical security hole in Microsoft Windows that was exploited by the Blaster worm last year.
Once located, Sophos says the worm infects the computer without asking the user's permission and hunts for traces of the MyDoom worms.
If a MyDoom infection is found, the Nachi-B worm attempts to remove it and download patches to fix the Microsoft vulnerability.
Graham Cluley, Sophos' senior technology consultant, said that while the virus author may think he is a modern-day Robin Hood, Nachi.B infects computers without permission, steals system resources and makes changes to a PC's setup and data.
He added that the worm can also cause unexpected conflicts on the user's PC.