Windows Defender stopped in W8.1

Brand new HP laptop, running W8.1 64, not yet connected to internet.

Uninstalled trial McAfee, then ran their special uninstall tool as well.

W Defender not (even) appearing in Services, & unable to turn on in Action Center, despite being able to see all program files in Explorer. Defender also greyed out in Control Panel, which says Application not found.

Tried:

1) double clicking file MSASCui.exe - says "WDef has been stopped, etc"

2) running sc Start WinDefend Enable in Command Line - no use, "cant find file"

3) in Services, stopping then re-starting Security Center service - no use, despite its apparent use for other members of this Forum

4) phoned Microsoft & lady other side of the world polite, but baffled, & promising to give me a call-back, escalating to Level 2

Any help, please ......cannot move on & connect to internet un-protected. There has to be a simple answer to turn W Defender back on.....

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