I'm not sure if you get that with iTunes or it's a separate application, but I doubt it has anything to do with blocking Windows Defender.
However, to eliminate it, go Start - type services.msc and press enter then look to see if it's listed.
If it is, check its status - does it show as Started and Auto or Started and Auto (Delayed) - or just Manual ?
If it's either of the first two, right click on it and select Properties then use the dropdown to change it to Disabled.
Click on Stop - Apply - OK
Scroll down to Windows Defender and check its status.
In my Win 7 it just shows as Manual but I also have Norton Security installed which may have disabled or turned it off.
Click on File/Exit then reboot to see if WD has started or can be started.
My money though would be on AVG.
This article says that from AVG 2011 Windows Defender isn't needed -
http://www.bestyoucanget.com/avgdefenderinstallDisable AVG for 10 - 15mins then reboot and try WD
https://support.avg.com/SupportArticleView?l=en&urlName=How-to-disable-AVG&supportType=homeIf it then starts, enable AVG.
Check to see if WD remains on and then reboot and check it again.