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I have the same problem with this topic! How to resolve BackgroundContainer.dll error on startup?
The thing is i did everything that says in answer but
i)I found nothing when i started regedit.exe and searched (F3 or CTRL+F) for BackgroundContainer.dll
ii)I made a full scan with AntiVirus Scanner like Microsoft Security Essentials/ Defender and also i scaned my PC with Malwarebytes' Anti-Malware,but i found nothing.
So this pop up message on start up keep appeared!
I know you will say that its a duplicate of this topic but when i did that question the deleted it and said if you have a question post it!Sorry about that.