win.ini is an .ini file which stored Microsoft Windows configuration data under Windows 3.1 through to current versions of Windows like Windows XP, although its use is now deprecated due to the creation of the Windows Registry, and is pretty much just a stub file. It exists in the root directory of the Windows installation (by default, C:\WINDOWS\).

Some other applications (such as calc.exe, the calculator shipped with Windows, and Cumberland Family Tree for Windows) use (or used) this file for storing their own data. The advantage of this for Calc is that a single byte of data (is the calculator in scientific mode?) does not take up an entire cluster of the hard disk; the advantage for Cumberland is that the settings (including expiry date of the free trial) are persistent across reinstallation of the software.

Thanks to C-Dawg for informing me that I should be using the word deprecated, not depreciated.