thanks. problem solved
1. disabled the windows driver signature.
2. uninstalled and re-installed the drivers, via Arduino IDE->Tools->Boards->"Arduino SAMD Boards (32-bits...)".
3. got the windows red alert window, asking if I want to install from an untrusted source.
4. I clicked "Yes".
5. Plugged the MKRZERO - it got recognized with a virtual COM port.