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\Microsoft\Windows\CurrentVersion\Uninstall\ and other product-relevant information is stored using the compressed GUID in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\.
Yet a google search to figure out what the compressed GUID actually is and why both forms are being used for saving product-specific data in the registry database only revealed algorithms for converting from one form to another.
Apparently, the compressed GUID is calculated from merely changing the order of the characters in a specific way, giving me more confusion about why this is the compressed form and why it is used this way.
Additionally, some sources appear to refer to the GUID as the product code of the software product, and others specifically use the term product code as a synonym for the compressed GUID.
I do not have much knowledge of the inner workings of Windows and its installers, but hope that someone can enlighten me with the information I was unable to find. I apologize in advance for my mediocre English.
Not sure what you are doing, or why this is a problem for your scenario. However, the GUIDs you mention - the rearranged GUIDs with the braces and dashes removed - are actually referred to as Packed GUIDs. Then there are also Compressed GUIDs that are just 23 characters long that are primarily used to create Darwin descriptors - which are combinations of the product code GUID, a feature name, and a component code GUID. They are used for MSI's advertisement features. This is according to Bob Baker's book "Getting Started with InstallShield Developer and Windows Installer Setups".
As far as I recall the packed GUIDs are apparently used to make registry searches more efficient. I am unfamiliar with the exact technical details involved. Perhaps Bob or Rob of WiX can elaborate.
I have to run, not really an answer, I will look at it again later. Please elaborate your question with more details on what the problem actually is. As a summary it seems the collective comments suggest that this GUID concept is due to registry space saving, searching efficiency and obfuscation.
My advice (if I understand correctly): do not attempt to read packed or compressed GUIDs from the registry directly - rather go via the MSI API (COM / Win32) - which should have the features you need to do almost anything with MSI.
Some Links:
MSI API Links (quite similar links):