The win.ini file is a Windows system file used with Microsoft Windows 3.x and 9x initialization that loads from the C:\Windows directory and loads settings each time Windows boots. For example, the communication drivers, wallpaper, screensaver, languages and fonts are loaded each time the win.ini initializes. If this file becomes corrupt or bad, Windows will either not load, or have several errors as it loads.
Windows XP does still look at a few entries in the win.ini, which is why it may still be present after upgrading to Windows XP. However, if you installed a new copy of Windows XP the win.ini file is blank or a very stripped down version. Later versions of Windows including Windows NT 4.0, 2000, Vista, 7, and later no longer have a win.ini. Instead of storing these settings in this file, they are stored in the system Registry.