Like all installers, MSIs want to register their product with the Add/Remove Applet, this registry is stored at "HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall".
Microsoft are obviously trying to hide the registry information so it won't be documented. I will try to add information here that I (or others) discover. Please check the information for validity yourself, that is don't blame me if its wrong!
Any information here may also be specific to specific versions of Windows Installer. It would be silly (to say the least) to use this information if the same information can be obtained via documented interfaces.
The "GUID Conversion" facility converts between the 2 formats of GUIDs that I know about.
The "PerUser.VBS" tool uses some of the information documented here to list products installed per-user.
I think others will only appear if one or more products have been installed per-user for that user.