Cannot start Windows Defender

[System]

My new acer aspire one came with windows 8 but had AVG installed. I couldn't start Windows Defender which I would rather have running. I first disbled AVG and tried to start Defender but no go. I un-installed AVG, retried and still no go. I tried to
set it to start automatically - the function was greyed out and unavailable. Help would be appreciated. Thanks - Basil.

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