I've both those applications, and haven't experienced any default file type changes.
It shouldn't be a problem to change any of them back.
Open any explorer window, select "tools\folder options\file types", and re-associate any file type with the usual program used to open it.
In this case, associate reg with registry editor again.
I've managed in the past, without any help from software, to screw a couple of these up.
I allowed the change to take place because Scotty kept barking at me when I kept saying "no" to the change. So, like you said, if I need to I can change it back.
In this case, it is a good thing that reg files are associated with Notepad. If you accidentally click a .reg file, it will open in Notepad for editing instead of being automatically merged into the registry. To install a .reg file, right click it and select merge from the menu. This behaviour can be a safety measure in case some rogue software tries to install a .reg file. The software won't be able to do so because of the new association and you'll get a visual clue that something is going on when Notepad suddenly pops up with a .reg file open. I don't know anything about WinPatrol but perhaps it did make the change for the reasons I mentioned.
In this case, it is a good thing that reg files are associated with Notepad. If you accidentally click a .reg file, it will open in Notepad for editing instead of being automatically merged into the registry. To install a .reg file, right click it and select merge from the menu. This behaviour can be a safety measure in case some rogue software tries to install a .reg file. The software won't be able to do so because of the new association and you'll get a visual clue that something is going on when Notepad suddenly pops up with a .reg file open. I don't know anything about WinPatrol but perhaps it did make the change for the reasons I mentioned.