Security experts have warned users to beware a new version of the prolific W32/Netsky virus.

According to McAfee's Avert antivirus research team, the Netsky.ag variant is a prolific mass-mailing worm that spreads via email, sending itself to addresses found on the victim's machine.

The company said last week that, due to an increase in prevalence, it had raised its risk assessment of the virus to 'medium'.

Netsky harvests addresses from local files and uses them in the 'From' field to send itself, Avert warned. It contains its own SMTP engine to construct outgoing messages.

This produces a message with a spoofed From address. When run, the worm displays a message box reading: "File corrupted replace this!"

The worm also copies itself to local folders containing string share or sharing, network shares and P2P shared folders. If PCs are infected users may notice unexpected network traffic.