In all flavours of Cumulus, a data sub-folder is created (normally below the folder where the executable is stored) to hold files listed here such as dayfile.txt and today.ini.
If you did not read the readme.txt that is displayed when you run the installer (for original Cumulus software), you might make a mistake about where you install the software on your Windows PC.
The read me text recommends that you install Cumulus 1 directly below the root on one of the drives (or partitions) on your Windows Personal Computer, as if you do that all the other files that Cumulus creates can be added in the same folder as the executable, or in sub-folders below it.
This mistake is most common if the operating system is Windows Vista/Windows 7/Windows 8 (or one of the corresponding server versions of Windows). The mistake is to install Cumulus.exe inside the Program Files hierarchy. If you make this mistake, then all the sub-folders that Cumulus uses will be installed in C:\Users\[username]\AppData\Local\VirtualStore\Program Files\Cumulus, because Windows operating systems do not grant write access to C:\Program Files except during installing, and therefore divert all attempts while software is running to create files or write data into a virtual store within the Local sub-folder of the hidden (by default) application data, per user, area. Unfortunately, the executable cannot tell that has happened, and so still looks for folders below its own directory.