What is a compressed GUID and why is it used?

Question

I know that in Windows, the GUID is used by the Windows Installer to check for already installed products under the registry key HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsof

Answer 1

The packed guid is reportedly used to save space in the registry. It is used for component Ids, so once you start getting a million components on the system I assume they were worried about the extra space usage. It's also possible it was used to prevent people trawling the registry to look for Windows Installer items instead of using the APIs. It's an undocumented implementation detail, and if too many people got into habit of using the registry instead of using the APIs there would be issues if that implementation changed. So treat this as interesting information, not something to use in an application.

Darwin Descriptors are different - they are just an encoding of product code, component id, and feature name. They occur in various places where that combination of data is useful for loading COM servers, starting programs etc. Again, it's undocumented, but if you wanted to untangle them you could LoadLibrary on msi.dll and find and call MsiDecomposeDescriptorW with the right parameters.