IIS provides a set of default error pages it displays when an error occurs. For example, it displays a standard error page when the requested document doesn't exist (the 404 error). While informative, the messages are unimaginative. You might want to add a little sizzle or humor to your site by customizing the error message pages. Or perhaps you want to offer additional help for users who experience specific problems. Whatever the case, it's easy to customize the error messages.
The HTML documents that IIS displays for standard errors are stored in systemroot/Help/iisHelp/common. You can determine which file is used for a specific error through the IIS console: