A Minnesota high school senior pleaded guilty Wednesday in federal court to unleashing a variant of the "Blaster" Internet worm, which crippled more than a million computers last summer.

Jeffrey L. Parson, 19, of Hopkins, Minn., could be sentenced to 18 months to three years in prison on his guilty plea to one count of intentionally causing or attempting to cause damage to a protected computer. He also could be ordered to pay millions of dollars in restitution, Assistant U.S. Attorney Annette L. Hayes said.

Parson pleaded not guilty after his arrest last August, but told U.S. District Judge Marsha J. Pechman on Wednesday, "I downloaded the original Blaster worm, modified it and sent it back out on the Internet."

Different versions of the Blaster worm, also known as the LovSan virus, crippled computer networks worldwide last summer. Parson's variant launched a distributed denial-of-service attack against a Microsoft Corp. Windows update Web site as well as personal computers. The government estimates that Parson's version affected more than 48,000 computers.

"We appreciate the fact that the defendant has accepted responsibility for the crime he committed," Microsoft's deputy general counsel, Nancy Anderson, said Wednesday.

One of Parson's lawyers, Carol Koller, said that Parson was young when he committed the attacks and that being arrested made him realize the seriousness of his crime.

"He has been exemplary," she said. "He has not touched a computer since the day of his arrest."

Parson has been out of jail on a $25,000 pretrial bond with electronic home monitoring. On Wednesday, Pechman -- citing his good behavior since the arrest -- ordered him taken off electronic home monitoring pending sentencing.

Still, Parson cannot leave his home except to go to work, or if supervised and approved by the court. The judge declined Koller's request to grant him greater freedoms, such as being able to go to the movies without his parents.

"Mr. Parson is not a normal teenager," Pechman said. "Mr. Parson isn't going to be like other teenagers, who can take the family car, go to parties, go to the beach. That's not the way it's going to be."