The pop-up window, used for person-to-person chat, pops up with a message from a 'friend', which links to an innnocent looking website.
Clicking on the link immediately infects user's PC with the virus, labelled Steckt.Evl by discoverers Trend Micro, The Daily Mail reports.
The virus instantly disables and removes anti-virus software, then spreads itself by opening chat windows on the Facebook friends of the user.
Security experts Trend Micro, which uncovered the new threat, said the worm propagates via instant messaging applications and social networking sites.
Like many other new Internet threats, the first thing the worm does is disable programs that might detect and destroy it.
According to the report, the purpose is to 'control' the PC, both to steal sensitive information such as bank details and to use the 'zombie' machine as part of a network of attackers in large-scale cyber attacks.