The tool msizap.exe that is available in the Windows SDK and elsewhere on the web (remember to always download from a trusted source) is a powerful but dangerous tool that is often used to quickly and casually, and can leave your machine in a corrupted state if not used correctly. The same is true for the Windows Installer CleanUp Utility which uses msizap.exe.

Windows Installer is a transactional, data-driven deployment technology used by most of the products deployed on Windows platforms today. At its core, it is a loose referential database that describes software applications and that information can be updated by changes, or transforms. That package may describe files, registry values, assemblies, and even custom data that is acted upon by the engine by first generating a script and then executing it. One of the final typical actions of this script is to register product and patch information so that it is cataloged and can be queried; and so that Windows Installer itself can perform future maintenance installations which includes updates and uninstalls.

Updates are supported by actions that first remove older data and then install the newer data. An uninstall essentially skips the install actions. That means, then, that files, registry values, assemblies, and custom data is simply removed.

With that in mind, msizap.exe merely removes the product registration information from the registry and optionally files in the Windows Installer cache – a critical set of files that is necessary for future maintenance installations including proper uninstalls. Removing files from your Windows Installer cache can cause products like the Microsoft .NET Framework 2.0 Service Pack 1 to fail to upgrade older versions.

Msizap.exe is not magic, but merely a tool that acts only on Windows Installer registration data. It will not run the removal actions. So, for example, if you zapped the entire .NET 2.0 RTM products instead of just certain patches, actions that unregister WMI providers incompatible with .NET 2.0 SP1 would not run and, therefore, would not remove the WMI provider registration. In addition, files and other registry data is not removed so the .NET Framework is not truly removed from the system. Msizap.exe is no more of an uninstaller than simply forgetting that a program is installed, which is all it basically causes the Windows Installer engine to do.

Msizap.exe is an effective tool when product registration is already corrupt for whatever reason; however, it should only be used as an absolute last resort. Always try to uninstall a product first through the Add/Remove Programs control panel or the Software Explorer for newer Windows platforms like Vista. If you cannot see the product listed in the control panel, see if there is an “Uninstall” icon or similar in your Start menu for that program group. If you don’t see such an item, search the web for what other people have done to uninstall a product. For Windows Installer products, by knowing certain data you can always uninstall from the command line using msiexec.exe. To help diagnose issues properly, always generate a log by passing /L*vx uninstall.log to the command line to msiexec.exe. In this log will almost always be the answer to determine what is wrong so that a product can be properly uninstalled.

For help uninstalling a product when errors are encountered, Aaron Stebner’s and my blog can be helpful most often for Visual Studio and .NET installation problems. At times, they may even help in general since problems like registration or cache corruption can happen to any product – especially if msizap.exe was already used improperly. You can also search for error strings you find in the Windows Installer log files which are most often right after the string “Return value 3”. The error message number like 1714 you will see is more helpful than a return code at the end of a log like 1603 (0x643).