NBC Universal, acknowledging that viewers are increasingly moving away from traditional television viewing, announced plans today for a service that will make popular NBC programs available to download free to personal computers and other devices.

The programs, including “Heroes” and “The Tonight Show With Jay Leno,” will be offered for a week immediately after their initial broadcasts. Commercials will be embedded in the programs and viewers will not be able to skip through them.

The move comes less than three weeks after NBC Universal announced it was severing ties with Apple Inc. after a dispute over how Apple was selling NBC programs on its popular iTunes service.

NBC first contracted with Amazon to offer its programs for sale to downloading devices like MP3 players. Now it is establishing its own downloading service, which NBC executives say they expect to become a viable competitor to iTunes.

“With the creation of this new service, we are acknowledging that now, more than ever, viewers want to be in control of how, when and where they consume their favorite entertainment,” said Vivi Zigler, the executive vice president of NBC Digital Entertainment. “Not only does this feature give them more control, but it also gives them a higher quality video experience.”

The NBC service, called NBC Direct, will begin a testing period in October with plans to be operational in November. The service will allow customers to download full episodes of NBC shows for seven days on Windows-based PCs. The file will expire after the seven days.

But NBC intends to transform the service into a model similar to iTunes by the middle of 2008 — that is, consumers will pay NBC directly to download episodes of the shows. “We did this to eliminate the middleman,” said Jeff Gaspin, the president of NBC’s digital division.