A computer worm that China warned Internet users against is an updated version of the Panda Burning Incense virus, which infected millions of PCs in the country three years ago, according to McAfee. A rootkit burrows into a system to try to hide the existence of malware. The first Panda worm gained fame in China for switching the icons of infected files with an image of a panda holding three incense sticks. The same image would also flash across a victim's screen, but the worm's final goal was to install password-stealing Trojan horses. The worm infected millions of PCs, according to Chinese state media. Its author was ordered to write a removal tool for the worm and later sentenced to four years in prison.

China's national virus response center warned about the updated worm earlier this week, but it dubbed the virus Worm_Piloyd.B and did not link it to Panda. The center said it had found a worm spreading online that infected executables and html files. The worm blocked a victim's PC from restoring infected files, turned off active antivirus software and directed the machine to Web sites to download Trojan horses and other malware, the center said. The center urged Internet users to step up defense on their PCs against unknown viruses.