Microsoft Corp.'s efforts to boost security in the upcoming Windows Vista, which will include built-in access controls similar to Linux, may also cause virus creators to look for greener fields elsewhere.

The number of viruses that has so far targeted Linux remains small compared to the thousands of viruses and billions of dollars in estimated damage and lost productivity caused by Windows viruses.

Some experts argue that because Linux, with its Unix heritage, was created from the ground up as a multi-user system with built-in access controls and privileges, it is fundamentally more secure than Windows. For instance, users on Linux generally do not run as the root or administrator user, unlike Windows XP. That limits the amount of damage a virus can cause to just those files and volumes accessible to the user, rather than to the entire computer or network.

Both Red Hat Inc. and Novell say they have enhanced those access controls, via their respective Security Enhanced Linux (SELinux) and AppArmor tweaks to the Linux kernel.

The relatively small number of Linux users spread among different versions of Linux has long hindered the growth of new software by creating a lower reward/effort ratio. That has also driven away virus creators, said Ed Metcalf, product marketing manager for McAfee Inc.