Because there are a vast number of permutations of computer environments, application setup programs must be quite sophisticated to work reliably in all situations. Whether performed by Windows Installer or other setup programs, all installer programs need to do a common set of tasks, and unless all are done correctly for the specific computer, the application will not run properly (if at all) or other applications or system services could fail.

Fundamental Tasks

When an application is installed, uninstalled, or updated (which can include anything ranging from minor patches to major upgrades), four fundamental actions take place according to a set of rules created by the developer who built the installation routine:

Create or delete directories and copy or delete files. All files containing the application’s executable code and data must be copied to the appropriate directories within the file system. This includes “shortcuts”-special files that are only links to other files and that provide users with desktop and Start menu access points to the application. During uninstallation, all files and directories no longer needed must be deleted. These operations are tricky because the developer must account for files that are in use, and therefore locked open, or files on which other applications are dependent.