Do any of the products you've tried say they've found something or nothing?
My thoughts:
Before you do anything further, ensure you have a copy of all your important data such as documents, pictures, music etc. You should have this anyway but now is the time to update it if not or it is out of date!
It may not be a nasty - you can have this issue if a DLL is corrupted (regardless of cause). In which case, you can normally rename the DLL (literally rename the file to msible.dll.old) and without refreshing Windows Explorer, scroll to the bottom to see if another one appears (it will be copied from C:\WINDOWS\system32\dllcache). If the DLL is locked (you cannot rename it), you can reboot in Safe Mode and rename it, then reboot in normal mode. It is not a core Windows DLL as I don't have it so it should not affect startup. See if this file is there.
Regardless of whether the issue re-occurs, next I would go here:
Avira Downloads
Download:
Avira AntiVir Removal Tool
...and run it to see what you get.
As said, you do need to remove the problem, and as you posted (how do does one do that?), you need to know exactly what to remove from where. Removing affected DLL(s) will probably not clear the problem - just the symptom(s).
After that, I would consider removing AVG and switching to Avira Personal (
www.free-av.com), installing that, updating it and running a FULL SCAN as well. AVG is reasonable but in my experience, not as good as Avira.
As you have run a FULL SCAN within an updated Malwarebytes, if both the above Avira products do not reveal any issues, I wouldn't necessarily be thinking it was a nasty but rather an underlying issue caused by something else (software is corrupting it, you might have disk corruption etc.).
Next I would be right clicking C: within Windows Explorer, choosing PROPERTIES > TOOLS > ERROR CHECKING > CHECK NOW and tick the 2 tick boxes, and then START. Agree to the prompt to schedule when Windows can have exclusive access to the disk and reboot.
See what that reveals. Any bad blocks, I would be thinking about replacing the hard disk. You can run on a disk with bad blocks but I don't recommend it - especially with the price of hard disks these days! You will need to rebuild if this is the case.
If not, and you still have the issue after all that, and given you have tried a restore to prior the issue starting (this would rule out a software update etc. causing it) - I would consider rebuilding from scratch myself.
PS: As a general rule of thumb, be very careful about installing tools and other software which 'can fix the problem'. You could end up installing duff software which gives you problems. A mainstream set of tools, like Avira and Malwarebytes, should be able to remove pretty much anything. Even zero-day problems are usually picked up by the heuristic abilities of Avira. Worse-case, find tools from trusted sources like Avira, Sophos etc.
