1. Can ERUNT be used manually? Meaning, I only use it to backup the registry before I install a new software. That also mean that I may used ERUNT manually on the same day a few times, if on that day, I installed a few software. I've configured to install Windows update manually and would like to backup the registry before I install any Windows update. I know that Windows update will create a Restore Point before it install but I can use ERUNT as a fall-back.
ERUNT can be used manually. I just don't see the need to run it several times a day. First off, I always create a restore point before installing any new software. This backs up not only registry settings but all of the system settings. So if something goes wrong I can restore those settings. ERUNT will not do that. It is just a registry backup.
I always create a restore point of my own before installing Windows Updates so if something goes wrong with the the update and the restore point that Update created doesn't work I will have a backup to fall back on...and I've never had a problem if I had to use that backup.
But the point is...if a software installation or a windows update messes up, using ERUNT to restore the registry will not restore the other system settings that the software installation or the update might have changed. And if Windows still boots up normally there is no need to restore the ERUNT registry backup.
System Restore is still you best option in those cases.
IF you make a change to the system, any change, and then it won't boot into normal windows, there is a possibility that something got changed in the registry that is preventing the normal boot up and assuming that you haven't been infected with malware or a virus (cause that can cause the system to stop booting also) then restoring the registry to the one that was working when you first booted up or backed it up makes sense. But you only need one registry backup for that day when you knew the system booted up OK.
2. What is the difference between ERUNT and ERUNTgui? I've googled and I found out they're different products.
ERUNTgui is just a graphical user interface to use with ERUNT.
ERUNTgui is a graphic user interface for the popular registry backup and restore program "ERUNT" created by Lars Hederer.
The reason for creating "ERUNTgui" is to place all the modules that make up the original "ERUNT" utility in to one user interface, making the program easier to use.
As far as I know the only ERUNT modules are ERUNT (used to back the registry up) and NTREGOPT. NTREGOPT is a registry optimizer that GTG does not recommend. We don't recommend any registry cleaners or optimizers. We've never found them to do anything to speed the system up and if used improperly they will break the registry and turn your computer into an expensive door stop.
I've never used ERUNTgui since ERUNT has all of the provisions for running it and restoring the registry if need be. And if windows won't boot up you couldn't get to it anyway so learning how to use the GUI (nothing more that a program window) wouldn't do you any good. You are better off learning how to use ERUNT the way the ERUNT documentation says to. I have the documentation printed out so that if I ever need to restore the ERUNT backup I will have it in front of me.
It just seems to me that you are thinking ERUNT will do something it was not designed to do.