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We looked at five apps that adopt this preventive strategy. Amust's 1-Defender and DropMyRights, two free programs, restrict the ability of software (and malware) to make major changes to your computer, such as in non-user-controlled parts of the Windows Registry. Though basic, these utilities are very effective--especially DropMyRights, which works with any program.

Stronger protection comes from two apps that wall off Internet programs in a "sandbox." Software running in the sandbox is blocked from making system-level changes and from accessing personal files, like bank documents in your personal finance app. GreenBorder Pro works only with Internet Explorer, though a Firefox version is planned; for $30 (the promotional rate at press time), you get a one-user license plus a one-year subscription that covers product updates. Fortres Grand's $50 Virtual Sandbox works with any program and must greenlight any process that wants to run on your computer; but its frequent alerts can grow irksome, and its setup is somewhat more complicated than GreenBorder Pro's.

For even more protection, consider the free VMWare Player and Browser Appliance. This hefty download supplies a Firefox browser that runs in a fully virtualized environment; it's much like using a separate PC just for the Web. There are some gotchas, but the player is fairly easy to install, and it offers a great deal of safety for systems with the resources to run it.