IDLE hands may be the devil's workshop, but an idle computer has the potential to seek out extraterrestrial life, crack encryption codes and maybe make a little extra cash for its owner.

Distributed computing, in which a large problem or daunting amount of data is parceled out to many -- even millions -- of computers to work on at once, is becoming a viable substitute for the once-almighty supercomputer. Even the humblest home machine can become a cog in a larger computational wheel, receiving data, churning through it at its own speed and returning the results. It's a digital-age illustration of the old adage about how to eat an elephant: one bite at a time.

By far the best-known distributed computing project is SETI@Home, a screen saver program, designed for home computers, that analyzes radio signals from the cosmos for patterns or other signs of alien life. Other projects have found million-digit prime numbers, deciphered secret codes and helped in the design of storage containers for nuclear waste.

Like many successful inventions originally created by scientists to share information and promote the greater good (including the World Wide Web), distributed computing is being examined for its commercial potential: computer owners may be able to, in effect, rent out unused processing time.

SETI@Home (SETI stands for Search for Extraterrestrial Intelligence) was developed at the University of California at Berkeley. It went into full distribution about a year ago and recently signed up its two millionth user. After a participant downloads the free screen saver program, packets of data recorded at a radio telescope in Puerto Rico are sent to the user's computer via the Internet. The participant's computer analyzes a packet when it is not handling other tasks, then sends the data back to the SETI@Home server and receives another packet.

Most of SETI@Home's users have probably never even noticed the computing time that they have donated to the project because the screen saver program makes great use of each computer's idle time. The program can be set to do its analyses when the computer has been idle for a while (for example, when the user takes a break). But it can also be set to function in the much smaller intervals of idle time that occur even when a machine appears to be in constant use.

''Let's say you're typing a story -- each time you hit a key, your computer works for about a hundredth of a second,'' said Dr. David P. Anderson, a computer scientist who has done extensive research in distributed computing and is also the project director of SETI@Home. ''And then it sits there for another second, waiting for you to hit the next key. So even while you think you are using your computer, about 99.99 percent of its time it's not actually doing anything useful.'' That is, unless it happens to be doing a little distributed work on the side.

Not all supersize projects are suitable for the distributed approach, and large computing chores like weather simulation do not work well because quick calculations are needed. One project in which a cluster proved effective was the RC5-56 Secret Key Challenge, in which thousands of volunteers donated computer time to crack a 56-bit encryption code. The project was one of a number run by, a nonprofit organization that has signed up about 60,000 participants to participate in a variety of code-cracking contests.

All told, SETI@Home's users are currently averaging about 12 teraflops, or 12 trillion calculations per second, over the course of a day, said Dr. Dan Werthimer, an astronomer at Berkeley and the chief scientist for the project. ''It's the largest computation that's ever been done -- on this planet, anyway,'' Dr. Werthimer said.

Dr. Werthimer calculated that home users had donated 283,000 years of computing time to the project so far. ''They're donating time, about 1,000 years every day,'' he said.

Supercomputers are usually used for large computing problems, like nuclear weapons research and weather forecasting, that require a staggering number of calculations, but they are ''super'' in both processing power and price tag. Computers capable of such power can cost millions of dollars, a price that is beyond the reach of many institutions. And a supercomputer, like any computer, can quickly become obsolete.

SETI researchers ran into the obsolescence problem before turning to distributed computing. They started out building specialized supercomputers to handle the data from the radio telescopes. Because of the rapid advance of technology, though, a SETI-specific supercomputer has a short life span. ''After five years, your machine is kind of a dinosaur,'' Dr. Werthimer explained, ''and it's time to build something new.''

Distributed computing has been around for several decades, but it has been greatly aided by two developments: the increasing speed of personal computers and the advent of the Internet, which provides an easy and almost instantaneous means of exchanging data between machines.

''The speed of the common PC has increased to such a dramatic rate,'' Dr. Anderson said, ''that the high-end PC today is as fast as one of these Cray supercomputers was about 10 years ago.''