Trojan pretends to do good

While Downloader.Lunii eliminates a variety of adware programs--often known as spyware--the Trojan also tries to maliciously change the security settings of Windows PCs and then downloads files from unknown Web sites, Symantec said.

"It's common that a Trojan will try to change as much security as possible," said Nigel Beighton, Symantec's director of enterprise strategy. "They often try to change Microsoft, Symantec and McAfee software."

Symantec classified Lunii as a low-risk Trojan.

Trojan horses are often referred to as spyware because of their ability to stealthily run in the background of a computer without being detected. Antispam organization Spamhaus recently warned that spyware has taken control of tens of thousands of networked computers. Spammers and hackers then use these "zombies" to launch denial-of-service and spam attacks, the company said.

Last week, the U.S. House of Representatives passed a bill to criminalize the act of altering PC configurations, taking control and downloading software onto a PC without the owner's consent.

Symantec has recommended users to switch off any unnecessary services--Windows functions such as printer and file sharing--and to keep security patches up to date.

Dan Ilett of ZDNet UK reported from London.