after a few days, It seems that it still happens, about 50% of the boot times.
I have run into this problem for 3 years. Two years ago, I installed another Win 7 on the same HDD to investigate more deeply. The second Win 7 has Avast installed and actually, it rarely has a black screen and the boot isnt really slow.
Here is the race between the two Win 7 on my laptop
https://www.youtube.com/watch?v=uxI7kgobTDgI don't know if my files or any specific can affect. On my main Win 7, I store documents on Documents folder of Windows, Music and Videos on C:\Media folder, some programs are installed on C, some on other partition. Will Avast emergency update cause? if it's true, why black screen doesn't happen on the second Win 7?