I just found someone will the same problem, and here is his conclusion :
Windows is blocking Eset temporarily when too many rules (UAC prompts) are created in a short time period.
I believe this is exactly my case, because I wanted a fresh start and so I cleared all of my HIPS rules.
Fired up some apps, got plenty of prompts, then I would go to the settings to create and change many rules.
All of these UAC prompts in a row are most likely the cause of my problem, which would explain why it comes and goes.
Do you guys know anything about this behavior? Can it be circumvented?