Till yesterday, I had an odd thing on my Win 8.1 netbook. Every time I used it on the internet, I got a vocal message saying that my computer has a virus and I must call a telephone number to solve that problem. And sometimes a blue screen!

No positive result with Avast. Then I tried Malwarebytes Anti-Malware. It found a lot of malwares on my PC and now, the problem is over. Only a small and short popup when a "virus" appears. Thank you MBAM!

  I've had the same thing, call this number, I keep Task Mgr at the bottom of the screen and have it end task.  I just rely on Windows Defender and its picked up a few things but won't let me download a lot of things.
Carl2