WINDHAM, Conn. — Until recently, Julie Amero says, she lived the quiet life of a small-town substitute teacher, with little knowledge of computers and even less about porn.

Now, she is in the middle of a criminal case that hinges on the intricacies of both, and it could put her behind bars for up to 40 years.

She was convicted last month of exposing seventh-grade students to pornography on her classroom computer. She contended the images were inadvertently thrust onto the screen by pornographers’ unseen spyware and adware programs.

Prosecutors dispute that. But her argument has made her a cause celebre among some technology experts, who say what happened to her could happen to anyone.

“I’m scared,” the 40-year-old Amero said. “I’m just beside myself over something I didn’t do.”

It all began in October 2004. Amero was assigned to a class at Kelly Middle School in Norwich, a city of around 37,000 people about 40 miles east of Hartford.

Amero says that before her class started, a teacher allowed her to send an e-mail to her husband. She says she used the computer and went to the bathroom, returning to find the permanent teacher gone and two students viewing a Web site on hair styles.

Amero says she chased the students away and started class. But later, she says, pornographic images started popping up on the computer screen by themselves. She says she tried to click the images off, but they kept returning, and she was under strict orders not to shut the computer off.

“I did everything I possibly could to keep them from seeing anything,” she says.