SETI@home's big launch comes more than a month after the first Unix versions were made available. About 12,000 people have been testing the software, and nearly 400,000 have signed up as volunteers. The concept behind SETI@home seems straightforward: Take radio signals from the biggest radio telescope in the world, carve the most promising data into thousands of pieces and pass it out in an orderly fashion to thousands of computers over the Internet. Each of those computers analyses a snippet of data for what could be the signature of an intentional transmission from a distant star system. If such a signature is found, that snippet is flagged for confirmation by professionals. It's taken more than three years to write the software. Over the past few months, the effort's organisers benefited from extra volunteer help as well as hefty contributions from the Planetary Society and several corporations. As a result, the first flavours of Unix software were made available to the public in early April via the SETI@home Web site -- setiathome.ssl.berkeley.edu. After weeks of beta testing, the Windows and Mac versions are being released to the downloading public Monday, SETI@home's organisers announced. Thirty varieties of Unix software will be on the site. Already, 200 years of computing time has been racked up by the 12,000 test users. But the acid test will come when the number of users rises to tens of thousands or hundreds of thousands. If even half of the 400,000 volunteers from 96 countries actually download and use the software, that would make SETI@home the world's largest experiment in distributed computing. "This project lets us do SETI a lot, lot faster, with 10 times more sensitivity and exploring more thoroughly the spectrum of radio frequencies we scan," said Dan Werthimer, the project scientist and a research physicist at the Space Sciences Laboratory at the University of California at Berkeley. "Plus, it's a fun and educational project, a global science project." But Werthimer realises that the project could lose data or go horribly awry if the load becomes too great. "If we ramp it up slowly, we can stop at some point if we have to," he said. Werthimer also reminded users not to raise the alarm if they come up against an unusual signal. Such signals would have to be double-checked by project staff to make sure they are not due to radio interference from Earth or orbiting satellites. "We're not asking people to call the press when they see a spike on the screen," Werthimer said. "We get strong signals all the time and have to sift through them."