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IDG News Service - After distributing a buggy antivirus update that apparently disabled hundreds of thousands of computers on Wednesday, McAfee is still at a loss to explain exactly what happened.

McAfee says that just a small fraction of its corporate customers -- less than 0.5 percent -- were affected by the glitch, which caused some Windows XP Service Pack 3 systems to crash and reboot repeatedly. McAfee blamed a bad virus definition update shipped out Wednesday morning, Pacific time, which ended up quarantining a critical Windows process called svchost.exe.

By the end of the day, the antivirus vendor still couldn't say exactly what caused the problem. "We're investigating how it was possible some customers were impacted and some not," said Joris Evers, a McAfee spokesman, speaking via instant message. One common factor amongst the victims of the glitch, however, is that they'd enabled a feature called "Scan Processes on Enable" in McAfee VirusScan software.

Added in version 8.7 of the product, this feature lets McAfee's malware scanner check processes in the computer's memory when it starts up. According to Evers, it is currently not enabled by default. However, some versions of VirusScan did ship with it enabled. McAfee's instructions for repairing affected computers can be found here.

A large number of users reported major problems after installing McAfee's bad update Wednesday.

Systems at Intel were knocked offline before the bad update could be stopped, according to Intel spokesman Chuck Mulloy. He couldn't say how many PCs were affected, but said that the problem was "significant."

"There were quite a few clients, laptops and PCs [affected]," he said. "We were able to get it stopped fairly early on, but clearly not soon enough."

About 40 percent of machines in Washington's Snohomish County were affected by the problem, according to John Storbeck, the county's engineering services supervisor. "This is a nightmare," he said in an e-mail message.

In Iowa, a local disaster response exercise was disrupted when 911 computer systems crashed, according to Deb Hale a Security Administrator with Internet Service provider Long Lines in Sioux City, Iowa. County IT staff soon started getting calls from other departments --- including police, fire and emergency response -- and began an emergency shutdown of all computers on the assumption that a virus was spreading.

After finishing the exercise, using a radio system for dispatch, participants learned that there was no virus, just a bad McAfee update, Hale said in a blog post. "Thanks to McAfee we were forced to test our response to a disaster while in the midst of a real 'disaster,'" she wrote.

According to reports Rhode Island Hospital, the National Science Foundation, and many universities were affected. Local police and government agencies in Kentucky experienced problems.