For laptops, it is essential to deploy antivirus protection and make sure it's kept up to date. In the very near future, I expect to see vendors release VPN tools that will require an up-to-date antivirus tool and personal firewall to be running in order to make a VPN connection. That'll force your users to keep their systems secure if they want to access the corporate network across the VPN. Until such products are released, however, a strong antivirus solution is your best bet.
For the handheld arena, there are some antivirus products for Palm OS and Pocket PC. This is a relatively new area, but vendors have some offerings such as McAfee's VirusScan PDA (which supports Pocket PC), Symantec AntiVirus for Handhelds (which supports Palm OS and Pocket PC), as well as several others.
I wouldn't make my operating system decision based merely on the malware possibilities of a given platform. Both Windows and Linux can be targets (although much more malware is written for Windows today). Still, you should (and likely will) make your OS decision based on business needs.
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This was first published in January 2004