This isn't the forum to request help with problems other than Forum problems. This is the Forum Feedback forum.
It may be that AVG is turning this off. AVG has it's own anti-spyware component and there is no need to have more than one anti-spyware scanner running at the same time. However, do keep using Windows Defender for manual scans every so often as a double check.
Mark
I have noticed that Windows Defender keeps turning itself off when I switch off the laptop. I have to turn it on everytime I switch the laptop on. It is an Acer Extensa 5235 running Windows Vista Business Edition. I also use AVG 9 antivirus.

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