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 among 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.