One particular offender is ByteFence. If you are seeing an error message like this, it's most likely ByteFence which should be removed from your system. (Bug Report Here)
Error details: Attempting to move successful download to C:/Users\User\AppData\Roaming\.minecraft\assets\objects\somethingsomethingsomething
The process cannot access the file because it is being used by another process
3. System Requirements
Starting from 1.12, Minecraft will no longer support Java 6 or 7. If you are using the latest launcher, you do not need to worry about this as Minecraft ships with its own version of Java. If you know you are using your own version of Java however, you'll need to make sure you're using Java 8.
The OpenGL requirement for 1.12 stays at OpenGL 1.3 (according to internal sources). Please report back if you have a OpenGL 1.3 only system that refuses to run Minecraft even at very low graphics settings.
Hardware requirements for 1.12 is currently staying the same as 1.11, but this may change in the future.
4. Graphics Glitching For No Reason After Upgrading
If your graphics suddenly started glitching since upgrading to 1.12, chances are that your graphics drivers are out of date.
Make sure you have the latest drivers installed and set VBO and VSync to On and set Mipmap to 0 in Video Settings (Options -> Video Settings -> Use VSync/Use VBO -> On).
If you are on a laptop with switchable graphics, you may also want to check that your computer is indeed using the dedicated graphics card.
I think it's pretty bad advice to suggest that players should "simply" uninstall a virus scanner when it gives them problems with Minecraft. What's going to keep their computers secured?
There's also no need for such drastic actions: most modern anti virus software provides a whitelist these days. Simply add the Minecraft launcher or, if that doesn't work, add the whole program (launcher) and/or gamedirectory (.minecraft) to the whitelist. After that the scanner won't mess with the game anymore.
I think that's a much more sensible suggestion than just advising people to uninstall the anti virus software, especially considering all the recent ransomware attacks happening.
also most of these "unsupported" anti-v are paid and you can't just uninstall and say nothing happens, go to the forums of your antivirus vendor if you have issues, they will help you, specially if it's paid