The three main GUIDs of a Windows Installer database are the Package Code, ProductCode, and UpgradeCode. The first is stored in the summary information stream (View menu in Orca), and the others are stored in the Property table. (Other forms of databases such as merge modules and patches have similar GUIDs in similar places, such as the merge module's GUID or the patch code GUID - each stored identically to the package code.)
To find them on a machine, you can look in the Uninstall key, where the ProductCode is often used. Or better yet, if you are looking to enumerate what is currently installed on the machine, you can call MsiEnumProducts.
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