Subscribers to America Online's (AOL's) email service are being warned to watch out for a password-stealing computer virus that is infecting users' machines worldwide.

According to antivirus software maker McAfee, the APStrojan.qu virus is the most active of a string of similar ones to affect AOL users over the last year. The company has rated it as medium risk.

The virus can be found in attachments within emails with a subject line that reads "hey you". The attachments are entitled "mine.zip", and the virus uses screen names found in infected users' AOL Buddy Lists. It only affects AOL version 4.0, however.

McAfee's chief executive Srivats Sampath said that APStrojan.qu installs itself on users' systems and tries to steal and forward account names and passwords. "What we're seeing now is an alarming increase in the number of infected computers as the virus replicates itself," he explained.

Instances of the virus have increased 100 per cent in the past 30 days, he added.

An AOL spokesman said the company is taking McAfee's alert seriously and is monitoring the situation. "We encourage people to be extremely careful in opening email attachments even if they are from familiar sources. Our top priority is protecting our members and we do that by educating them," he added.

The virus is written in Microsoft's Visual Basic 5 and first appeared in a slightly different form in January last year. A free scan and fix is available at aol.mcafee.com.