As Microsoft's operating system for personal digital assistants (PDAs) becomes more popular with enterprises, the potential threat of computer viruses affecting them is more real. Anti-virus experts say PDA viruses will pose a larger threat as PDAs that can surf the Web and send and receive e-mail become more common. Handhelds could be new paths for viruses and malicious code attempting to enter corporate networks. While handheld computers were once favoured only by technology geeks, the devices are drawing more users as they become easier to use and deliver more capabilities such as Web browsing, e-mail, instant messaging and gaming. In particular, devices running Microsoft's Pocket PC 2002 operating system (OS) are slowly becoming popular with enterprises as they explore wireless connectivity through the use of PDAs for business purposes. Lee Fisher, an anti-virus specialist from McAfee, says future malicious code is likely to expand attacks to include other forms of popular Internet communication such as instant messaging as well as infect mobile devices such as PDAs connected to the network. He says in a year, mobile devices such as Palm and Pocket PC handheld computers connected to networks will become vulnerable to attacks. To date, only a handful of viruses have targeted Palm-powered handhelds and damage has been limited. However, last August, the Liberty virus appeared, disguised as an emulator for Nintendo's GameBoy that claimed to let users crack game security codes. The virus erased all programs and data on the PDA. Another two viruses - Vapor and Phage 1.0 - blackened screens on some Palm devices. In 2000, a virus writer angry with Spain's Telefonica telecom company created a variant of the Love Bug virus that spammed thousands of mobile phones by sending a flood of messages to the phone monopoly's e-mail-to-phone gateways. While the attack showed the possibilities for spreading viruses to Internet-connected devices, currently only a fraction of handheld and mobile phone users are online, making the synchronisation process the most likely avenue for the spread of malicious code. Mr Fisher points out that even if viruses were not transmitted wirelessly to PDAs via e-mail, users can easily download a virus in a Word or Excel file to their PDA from their PC and then introduce the virus into their corporate network when they synchronise the device with their PC at work. 'Handheld devices can be as dangerous to a corporate network as the floppy disk,' he says. 'As virus writers become more versed in writing for that wireless environment, offering protection against viruses delivered over the airwaves will become critical.' Trend Micro ( www.antivirus.com ) provides a free program, PC-cillin for Wireless, which deactivates viruses that infect PDAs. McAfee ( www.mcafee.com ) and Symantec ( www.symantec.com ) also sell anti-virus software for Palm OS and PocketPC.