I do at-home PC support and also help people with Macs although I am far from a Mac expert. Helping a client with a brand new iMac yesterday and it was acting like a PC with a virus. Scenario is that her nephew helped her set it up and downloaded Office software from somewhere (we don't know where but he's a college student so could have been perfectly legit but all bets are off) and then her 3 little kids played with the Mac. I was there to help set up user accounts for the kids with parental control, etc.
I have never seen a Mac behave the way this one did. Basically the system randomly refused to connect to some web pages, then when it would connect links would only open halfway and then Safari would simply close with no warning or error message. Additionally sometime links took me to the wrong page, but not advertising pages, just the wrong page within a website, i.e. clicking on an ESPN story would take me to a different ESPN story than the one clicked on. This went on through several reboots, rebooting the router, resetting the router, etc. Other systems on the same network were fine. After about 15 minutes of frustration I simply restored the iMac back to factory and after that had no issues.
Any clue what i was dealing with? It was easy enough to restore a new system but not sure what to do if I run into these symptoms again.