Typically (though not universally) if a piece of software uses MSI-based installation the GUID can be found in the Uninstall entry. It will usually either be the key name or will appear in the UninstallString and/or UninstallPath value. Sometimes life is easy and there is a ProductGuid value.
Uninstall entries can be found here:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall
On a 64-bit version of Windows there are two such keys, one for 64-bit software and a second for 32-bit software:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall
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