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To contribute to a worthy cause like cancer research, you used to have to part with something you might miss--like your cash. That's no longer the case. You can now donate something that would otherwise be wasted--your CPU's unused processing power--to any number of important scientific or medical projects. But should you? And if so, which project should you choose?

The questions are especially timely now that Intel has thrown its weight--and $1 million--behind one such endeavor, the United Devices cancer research program, developed in conjunction with Oxford University.

The Intel program, launched in April, hopes to recruit 6 million people to download its client software. Their PCs will provide 50 terraflops of collective processing power (1 terraflop equals 1 trillion floating-point computations per second). "That's ten times bigger than the world's largest supercomputer ... for less than 1 percent the cost," says Pat Gelsinger, Intel's chief technical officer.

The Intel project is one of several that work like this. You install client software on your PC, where it runs in the background either as a screen saver or during the CPU's so-called "idle" cycles. It grabs problems from the Web site and reports back results over the Internet. And the collective power of thousands of screen savers at work gives such distributed computing networks their supercomputer-like muscle.

The similarity ends there. Distributed computing works best on "embarrassingly parallel" problems, those that can be broken down into many small independent calculations. It's less effective when individual calculations affect each other. It fails, for example, at weather forecasting, where a perturbation in one part of the atmosphere disturbs its neighbors. Many other supercomputer problems--like modeling airflow around a wing or the physics of a bomb blast--lie beyond distributed computing's reach.

Of course, plenty of important work remains. SETI@Home, the granddaddy of distributed computing projects with nearly 3 million participants, is searching for intelligent life in space. United Devices, which wrote the Intel client, is also investigating the human genome. And Entropia, a key competitor, is seeking treatments for AIDS and clues to stock market volatility.

Distributed computing can also help solve many stubborn problems of allocating scarce resources--like scheduling fleets of container ships or salespeople. "Nobody thinks of container-ship modeling as a classic problem for distributed computing, but it works well nonetheless," says Marc Hedlund, cofounder of Popular Power, which earlier this year closed its doors as a commercial entity but still conducts flu vaccine research.

How do you decide which project to support? First, ask yourself whether you want to support commercial distributed computing projects, like those run by United Devices and Entropia. (At United Devices, you can indicate that you want to do only scientific work.)

Security is another concern. The distributed computing client software, like any program that communicates over the Internet, could be a target for hackers. Your best protection may be to stick with well-established firms.

Leaving security aside, the best way to choose a charity is to put your CPU where your heart is. Senior Associate Editor Richard Baguley, whose mother died of cancer last year, has set up a PC World team on the Intel cancer research site (for Richard's list of this and other projects, see the next page).

Others may be drawn to the AIDS project, to SETI, or to something else. Wherever you place your bet, though, expect distributed computing to keep growing. "I hope it eventually becomes so common that we take it for granted, so that the portfolio analysis you run on your desktop may use 20,000 other computers for an hour," says Scott Kurowski, Entropia's founder. "That could be the most powerful Internet resource ever created."

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