installer

In the above two cases, the file check is delayed until the next start, because some setup programs will realize that the file is in use and cannot be replaced, so they use Exit­Windows­Exec to exit Windows back to MS-DOS, run a batch file, and then start Windows back up. We have to wait until the restart of Windows to catch any files that were improperly modified by the batch file.

— Raymond Chen, Microsoft veteran, describing how Windows 95 detected setup files, as quoted in the Neowin article.