An instant messaging worm, called "Bizex", targets ICQ users with invitations to visit a site and then installs malicious code on the target computer.
Russian anti-virus company Kaspersky Labs issued a warning, and estimated that 50,000 PCs worldwide have been affected.
However, a Symantec security researcher said his company has seen only a handful of Bizex infections and ranks the virus's threat level as low.
"We've seen five infections so far," said Brian Dunphy, a Symantec director. "It's one of several new viruses we've picked up today."
The Bizex worm sends a message directing recipients to a website called "jokeworld", according to Kaspersky Labs' advisory. Once the site is loaded, Bizex begins infecting the target computer, accessing the recipient's ICQ list to send copies of itself to other users and scanning the infected computer for information on installed payment systems.
It appears that the worm attempts to make money by stealing financial information - but the attempt appears to have been short-lived, Kaspersky Labs said. The security company said the jokeworld website closed down just four hours after the worm made its debut.
Because Bizex requires users to click on a link before any damage can occur, avoiding infection is easy if users follow the precaution of not clicking on unknown links, Dunphy said.
Stacey Cowley writes for IDG News Service