Further variants of thel Sober worm could still infect Windows-based computers warn anti-virus firms.

Both Kaspersky Lab and Symantec have detected worm variants. Kaspersky noted three variants of E-mail-Worm.Win32.Sober, which Symantec identified as [email protected]

The variants are modifications of the same program, said Kaspersky. A "large number of samples" of the variants have been intercepted in e-mail traffic, indicating that the worms are spreading by spam containing infected messages.

The messages might not have a subject line or text, but can be identified by the attachment name. The attachment names thus far identified are: Exceltab-packed_list.exe; Liste.zip; Reg-List-Dat_Packer2.exe; reg_text.zip; Word-Text.zip; Word-Text_packedList.exe; Word-Text_packedList.zip.

The worm activates only if a computer user clicks on the attachment, which causes a false error message, "WinZip Self-Extractor. WinZip_Data_Module is missing ~Error," to pop up, Kaspersky said. The worm variants copy themselves to the Windows system directory and then registers the files to the system registry so that the worm launches every time Windows is rebooted.

The worm uses its own SMTP (Simple Mail Transfer Protocol) engine to spread, Symantec said. Spam that it generates is in either English or German, said Symantec. Instructions for removing the worm from infected systems can be found at Symantec's website by clicking on the Sober variant listing under "latest threats."