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danomac writes:

Apparently a bug in a Microsoft Security Essentials update caused XP computers to bluescreen. After rebooting once updates were installed the XP machines would bluescreen with "MsMpEng.exe application error".

Microsoft has fixed the error by releasing a definitions update for Security Essentials. From the article:

Many point-of-sale systems, which some businesses are protecting using Microsoft Security Essentials (MSE) as a way to meet the antivirus requirement of the Payment Card Industry PCI), still use Windows XP or related operating systems, including Windows XP Professional for Embedded Systems and Windows Embedded POSReady 2009. Those systems were affected by the update as well, according to one New England value-added reseller with more than 500 clients in the hospitality industry.

While Microsoft corrected the issue within days, the bug crashed at least hundreds of machines. For the New England firm, the issue affected more than 250 machines at 50 customers who relied on the systems, a consultant at the company said on condition of anonymity. "This affected about one half of our customers running Windows XP," he told eWEEK. "This brought their business to a 100 percent standstill until we could resolve the situation. In a pinch, the only solution we could determine was to uninstall MS Essentials to get them running their business again."

While uninstalling Microsoft Security Essentials worked around the issue, it also caused an additional problem: Even though Microsoft later fixed the update, MSE could not be reinstalled on Windows XP computers because the systems are no longer supported by Microsoft, the source said.