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.