The popular file-swapping system Kazaa has been hit by a computer virus.

A malicious computer program, dubbed Benjamin, has been found that spreads by infecting the machines of people who use the system.

Computers infected by Benjamin will find it increasingly difficult to swap data and could find large parts of their hard disk taken up by copies of the virus.

However, anti-virus companies say the program is not too malicious and many have already produced cures for the infection.

Clogged disks

When it first infects a machine, Benjamin creates a directory and copies lots, sometimes thousands, of versions of itself into it.

Even movies can be found on file-sharing networks

The versions have different names based on a list held by the virus.

The directory it creates is open to other Kazaa members and anyone who downloads any of the disguised virus files will become infected.

As well as taking up lots of space on a hard disk, the virus also jams net connections and gradually makes it impossible to swap files via Kazaa.

The malicious file was first discovered by anti-virus firm Kaspersky Labs on 18 May.

Duping users

Kazaa, like Napster, lets members swap the music, image and video files on their computers with other people on the system.

As a peer-to-peer system, Kazaa does not have a central server that gives out files to users. Instead, the net links connections and machines of people who participate in the network.

According to Sharman networks, which owns Kazaa, the software to use the swapping system has been downloaded more than 81 million times.

Kazaa is not the first peer-to-peer system to be hit by a virus.

In February, a worm was discovered that tried to spread via the Gnutella network.

Before now, virus writers have tended to concentrate their efforts on e-mail viruses that dupe people into opening them.

However, anti-virus companies say that any popular program is a tempting target for virus writers.