The Trojan horse program decided to battle against its much-larger rival, by stealing data and then removing the malicious program from infected computers.

 According to the security researchers, the relatively unknown Spy Eye toolkit added this feature called "Kill Zeus” just a few days ago in a bid to displace its larger rival.

"Kill Zeus” apparently removes the Zeus software from the victim's PC, giving Spy Eye exclusive access to usernames and passwords.

Zeus and Spy Eye, both Trojan-making toolkits, designed to let abusers easily set up their own "botnet" networks of password-stealing programs.

These programs emerged as a major problem in 2009, with the US Federal Bureau of Investigation estimating last October that they have caused US$100 million in losses.

 Trojans such as Zeus and Spy Eye steal online banking credentials. This information is then used to empty bank accounts by transferring funds to so-called money mules US residents with bank accounts who then move the cash out of the country.

Sensing an opportunity, a number of similar Trojans have emerged recently, including Filon, Clod and Bugat, which was discovered just last month.

Besides, Spy Eye claims to be the most aggressive crimeware with its "Kill Zeus" option. The software can also steal data as it is transferred back to a Zeus command-and-control server.