(AP) -- Microsoft Corp. says it's not to blame for a problem that is causing a "limited" number of Windows computers to boot up to a blank black screen.
The problem is cropping up on computers running the new Windows 7 software as well as Windows XP and Vista.
Microsoft says its customer service department doesn't consider the black screen problem a "broad customer issue." It hasn't identified the problem but says some malicious programs on the Internet are known to cause black screens.
A British security company called Prevx says 50,000 people have downloaded a tool it offers to fix the problem.
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