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Simply, an uninstaller is the opposite of an installer. During the uninstallation, the uninstaller removes all the components which were installed. Most of the software vendors ship an uninstaller with their application.
A Third-party uninstaller is a program that uninstalls other programs. Being a third-party uninstaller, it should not be specific to any program (i.e., it should work with (be able to uninstall) many, if not every, program -- which is very difficult in practice, given very broad range of programs).
Generally, most (third-party) uninstallers contains the following components:
- Logger: The Logger is used to log installations (e.g., log which files were added or changed, which registry entries were added or changed, etc. at the time of installation). This log is used when the user decides to uninstall the logged installation at later date (in that case, the log is "reversed" -- i.e., the log is read, but opposite actions are taken in opposite order),
- Uninstaller The Uninstaller is used to reverse changes in the log (i.e., the log is read, but opposite actions are taken in opposite order based on the log). This way, the applications can be uninstalled because all changes that were made at the times of installation are reversed.
- Analyzer (optional) The Analyzer is used to uninstall programs of which installation is not logged by the uninstaller. In that case, the program analyzes the program and finds (and deletes, if the user decided to uninstall the program) all related files and registry entries,
- Watcher (optional) The Watcher watchs running programs for installation programs (and usually offers to start the logger when such programs are detected). Usually, this works by watching the tasklist for any names that are usually used by installation programs (e.g., SETUP.EXE, INSTALL.EXE, etc.)
- Other tools (optional) Some uninstallers may also contains other related tools like a junk file cleaner, browser history cleaner, cache cleaner, etc. These tools are not mandatory for uninstallers and are added mainly to improve the merchantability (since pure uninstallers are unpopular today).
Usually, in most (third-party) uninstallers, the following operations can be performed:
- Log: watches any changes made to the system during installation,
- Uninstall: uninstalls a program (based on the log or analysis),
- Move/Transport: backs up a program (usually packs all required files and registry entries in a "transport package", which can be transported and installed in another computer) and uninstalls them,
- Archive: backs up a program (and possibly pack them, to save disk space) and uninstalls them,
- Backup: backs up a program (for reinstallation at a later date should the program fail),
- Other related operations: like junk file cleaning, cache cleaning, etc.
Third-party uninstallers today
Nowadays, pure third-party uninstallers (uninstallers that only contain basic components -- that is the watcher, logger, uninstaller, and probably the analyzer) generally are unpopular and are not needed anymore, because:
- Most software programs' come with their own uninstallers (which is obviously better and more accurate),
- Most uninstallers are not entirely accurate (most of them are known to leave leftovers, which should be deleted by uninstallers),
- There are many better and safer tools that can be used to replace uninstallers,
Because of this, to make uninstallers more merchantable, most (if not all) of today's uninstallers contain other related tools (such as a cache cleaner, a junk file cleaner, etc.)
List of third-party uninstallers
- Advanced Uninstaller Pro 
- Data Becker Power Cleaner ,
- McAfee Uninstaller (formerly CyberMedia Uninstaller and MicroHelp Uninstaller , written by Ken Spreitzer ),
- Norton CleanSweep ,
- IMSI Uninstaller .
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