The win.ini file is a type of initialization and configuration file in the Windows operating system that stores basic settings at the time of booting.
It is a file associated with legacy Windows OS—initiated in Windows 3.x and continuing up to Windows 9x, with some backward compatibility support in Windows XP.
Techopedia explains Win.ini
Win.ini was used to store and load some of the basic and core settings required at startup for a Windows machine. This typically included communication drivers, languages, fonts, screensavers, wallpapers, etc. Any settings made to such services were immediately saved on the win.ini file. When the computer was started/restarted, Windows loaded and extracted information for user-defined settings from the win.ini file.
Windows XP has some support for it, only to provide compatibility with applications that were built on the older 16-bit framework. Win.ini was phased out in favor of Windows Registry and was completely removed from Windows 7/8.