“YOUR MICROSOFT COMPUTER HAS BEEN BLOCKED” is a misleading ads. Simply ignore it. Its possible that your computer is infected with an adware, so I suggests you to check your PC. Use the following free tools:
AdwCleaner by Xplode from http://www.myantispyware.com/download/adwcleaner
MalwareBytes Anti-Malware from http://www.myantispyware.com/download/malwarebytes-anti-malware
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