BURBANK, Calif. (AP) -- 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 on television. The file will contain embedded 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.

High-definition shows will eventually be available through peer-to-peer technology, which will speed the download of larger files, NBC said. A special software player with filtering technology will prevent users from playing illegally downloaded videos.

NBC, like many networks, already sells episodes for permanent download and streams them for free on its Web site.

The network is experimenting with a variety of business models and technology to give viewers more options for watching shows.

That goal has become more important since NBC Universal, a unit of General Electric Co., said it would no longer sell TV episodes on iTunes because it wants the ability to sell its shows at a variety of prices instead of the $1.99 standard enforced by Apple.

''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.

Other programs that will be available at launch include ''Life,'' ''Bionic Woman,'' ''30 Rock,'' ''Friday Night Lights,'' ''Late Night with Conan O'Brien'' and ''The Tonight Show with Jay Leno.''