BURBANK, Calif. &#8212 NBC will start offering limited free downloads of shows such as “Heroes” and “The Office” as part of its bid to expand the digital distribution of its programs and compete with Apple Inc.’s iTunes Store, the network announced Wednesday.

The new service, dubbed “NBC Direct,” will allow users to download episodes to computers running Microsoft Windows software for up to a week after the show has aired. The file will contain advertising that cannot be skipped.

Seven days after the episode’s TV debut, the digital file will expire.

The new service will launch in a test version in October and be expanded in coming months to let users subscribe to shows and download them automatically. Upcoming versions to be introduced by year’s end will also allow viewers to move the file to a portable viewing device or watch them on a Mac computer.

“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 consumer their favorite entertainment,” said Vivi Zigler, executive vice president of NBC Digital Entertainment.