Settings and configuration files of XnView Classic are explained in much detail in topic Info about XnView's settings (store & load)
XnView MP stores all its settings in configurations files. These configuration files are:
xnview.ini - all program settings
default.bar - toolbar definition: icons and commands
default.key - keyboard assignments: shortcuts
Resetting XnView MP's program configuration
For various reasons a reset of the program configuration might be needed. All default settings are "burnt" into the program code of XnView MP. So you can reset XnView MP's program configuration by simply removing the configuration file xnview.ini.
Use the following steps to reset XnView's program configuration:
1. First, locate the xnview.ini . For this, please start XnView and select button "Open Config files location" in Tools » Settings... | Interface | Integration.
2. You'll now see file explorer (Windows Explorer) displaying the directory with the file "xnview.ini"
3. Exit XnView
4. You can either delete "xnview.ini" file or rename it. For problem analysis it is recommended to rename it, e.g. to "xnview.ini.bak"
Now, you're done, already. Next time you start XnView it will use its burnt-in defaults.