Microsoft is playing down a massive hacker attack that caused computers running Windows NT software across the United States to crash.

Thousands of Americans found their computers frozen from an assault launched over the Internet.

Microsoft has provided a software patch to protect computers running Windows NT, a popular operating system for larger computers and networks.

Windows popularity means any attack has massive effect

The attacks, which started on Monday, hit many government offices, including nine of Nasa's 10 major field offices. There were similar assaults on Tuesday and Wednesday.

Universities, including the Massachusetts Institute of Technology (MIT) and the University of California at Berkeley, also were affected.

There were no reports of any computer information being lost or stolen during the attacks.

Computer experts believe the attack preys on Windows NT systems by sending a barrage of invalid data to computers. The computer devotes more and more memory and processing power to the invalid data until it crashes.

The patch, which has been available since January, works by stopping computers from reacting to the flawed data.

Jeffrey Schiller, a network manager at MIT, said the hacker exploited a bug in the Windows 95 and Windows NT operating systems known as "Boink," "Teardrop II" or "New Tear."

Ron Broersma, a computer security expert with the Space and Naval Warfare Systems Command in San Diego, said thousands of desktop computers used by the US Navy crashed on Monday.

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The Microsoft remedy was applied in time to avert a second attack on Wednesday, he said.

Craig Huckabee, a research associate at the University of Wisconsin's computer lab, where more than 160 computers were hit, said the attacks appeared to be a prank.

"It could be just to draw attention to Windows NT or some of the failures in Microsoft products," he said.

But Mr Schiller said the incident underscored the vulnerability of networked computers.

"What you're dealing with here is someone who is actively trying to find a way to make your program misbehave. To me the Internet is a hostile place, and we have to build our programs to deal with that."

Microsoft is downplaying speculation that the attack was timed to coincide with its chairman Bill Gates' going in front of a Senate Judiciary Committee on Tuesday.

Mr Gates was defending the company against accusations that it uses its near-monopolistic power to kill competition.

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