ANI files are used to store default Windows animated cursors, such as the spinning hourglass. They can also be used for storing custom animated cursors. ANI files can be downloaded from third-party developers and are often included with custom desktop themes.
NOTE: On March 31, 2007, Microsoft issued a Security Advisory regarding a vulnerability in the way Windows handles animated cursor files. The issue affects Windows 2000 through Windows Vista. A Security Update is available on the "Microsoft Security Bulletin MS07-017" Web page.