If only it had been that easy. McVirus "Turns Off" Defender and you actually have to un-install McVirus before you can "Turn On" Defender.Click on windows icon > settings > Update & Securty. Windows Defender should be the second one listed, if it's installed.
I would highly recommend a clean install of win 10 though. I use Avast Free. Have for years.
I thought long and hard about formatting and doing a clean install. But after doing some reading and talking with a friend who does Desktop Support for a living I decided to keep the Lenovo apps and just remove the Windows Bloat. There is a lot of great info on Win 10 and how to deal with it out already, even found a forum with a whole bunch of useful info.
I'm up, configured, de-bloated, and coming to terms with it...MS screwed the pooch on a whole bunch of stuff here. The worst by far is Windows Update.