You reminded me of the horrible Fake Antivirus tool -Security Tool,i surfed about 3 hours that day to remove this crap.There were some methods like manually changing registry values etc,but i can't dare to do anything with registry files,Finally i discovered this easy method,So follow these easiest steps below for successful removal:
1) Download MalwareBytes Anti malware (It's FREE),Don't run it,Just save it on your Desktop.
2) Shutdown your computer,now start your computer in Safe mode (I think you know how to start in Safe Mode

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3) Now locate the Malwarebytes setup on your desktop and run it,run it for full scan,Full scan may take about 45 minutes to 1 hour,just don't worry.
4) It will locate some errorenous/harmful files in result,Just remove them.
5) Enjoy normal surfing.