Usually trird-party apps (or their installers) use notepad.exe from system32 for opening its text files (readmes, changelogs, configuration files and other), but exist some installers, which use the same notepad, but from SystemRoot (folder where system is installed, usually C:\ReactOS or C:\WINDOWS). Such copy of notepad is also present in Win2k3, and that installers open its readmes succesfully there. But in ReactOS it's absent, and they fail to do it. For example, Battlefield 1942 Single Player Demo installer fails to open it, and Doom 3 Demo installer even proposes to rollback the installation after unsuccessfull opening of readme at the end of setup (see screenshots).
I see only one solution for this problem: duplicate notepad.exe, which we already have in system32, in SystemRoot, via CMakeLists.txt of notepad's sources, which I did. After my fix, the second copy of notepad.exe appeared in C:\ReactOS, and at least installers mentioned above, opens its readmes correctly.
P.S.: Although in Win2k3 that notepad is named in uppercase (NOTEPAD.EXE), in ROS with my fix, it's named in lowercase (notepad.exe), but nevertheless this doesn't interfere apps to open text files, and my changes fix the described problem.
P.P.S.: I checked now: in Win2k3, the versions of two notepads are completely match, so it's not required to change the version of our notepad.