Recently I experienced internet connection problems on my pc - my laptop and Ipad were OK. The connection problem effectively blocked the pc from the internet though for a short period it appeared to come back to life.
Ran NIS, W Defender that indicated all was well. It was then I discovered the above s/w and on checking it out on the net with my laptop found it to be malware. This effectively was a DNOS on my PC.
On de-installing via the usual way in the Control Panel - the problem was gone.
I hope this may be of help.
Could Norton advise.